Allow downloading PDFs instead of opening them in Brave.

[mkarolin]

Fixes brave/brave-browser#1531

The fix does the following:

1. Makes the initial decision on whether to load PDFJS extension based
   on the value of kPluginsAlwaysOpenPdfExternally profile preference in
   addition to the command line switch.

2. Watches profile preference value kPluginsAlwaysOpenPdfExternally and
   adds/removes PDFJS component when the value changes.

3. Modifies js behind the chrome://settings/content/pdfDocuments so that
   if the PDFJS extension is disabled from the command line the option to
   download PDF files instead of opening them in Brave is set to ON and the
   toggle is disabled.

4. Adds an alternative call for whitelisted extensions in
   extensions/common/manifest_handlers/mime_types_handler.h that removes
   Chrome's PDF Extension ID from whitelisted IDs and patches
   chrome/browser/plugins/plugin_utils.cc to use the alternative call. This
   is done because on Linux that extension is not removed from profile
   enabled extensions (because there is no extensions install verification)
   and it ends up being picked as a handler for PDFs. If our PDFJS extension
   is disabled via command line, this causes a PDF to be neither displayed
   nor downloaded.

Note, that this fix doesn't address being able to turn off opening PDFs
in Brave in Guest/Tor profiles. The web ui setting to do so is not
available in these profiles.

The command line switch to not load PDFJS already applies to all profile
types.

Adds browser tests to check that Chromium's PDF extension is not
considered for handling PDFs and if the PDFJS extension is not loaded or
disabled from command line a download is initiated when navigating to a
PDF URL. Also adds test to verify that when PDF download preference is
toggled the component loader will take an appropriate action (add or
remove PDFJS extension).

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Verified that these changes build without errors on
  • Windows
  • macOS
  • Linux
• Verified that these changes pass automated tests (npm test brave_unit_tests && npm test brave_browser_tests) on
  • Windows
  • macOS
  • Linux
• Ran git rebase master (if needed).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed.
• Add appropriate QA labels (QA/Yes or QA/No) to include the closed issue in milestone

Test Plan:

Automated testing
npm run test -- brave_browser_tests --filter=BravePDFDownloadTest.*
npm run test -- brave_browser_tests --filter=BravePDFExtensionTest.ToggleDownloadPDFs
npm run test -- brave_browser_tests --filter=BravePDFExtensionDisabledTest.ToggleDownloadPDFs

Manual testing

1. Test that PDF is downloaded if the Download PDF files... setting is turned on

• Start Brave browser and navigate to chrome://settings/content/pdfDocuments,
• Toggle the Download PDF files... setting to ON,
• In a new tab, navigate to a PDF URL (e.g. http://www.pdf995.com/samples/pdf.pdf),
• Verify that the PDF is not shown in Brave and is instead being downloaded,
• In the settings tab, toggle the switch back to OFF,
• In a new tab, navigate to the same PDF URL,
• Verify that the PDF is now show in Brave.

2. Test that command line switch is correctly interpreted in the settings UI

• Start Brave browser from the command line with the --disable-pdfjs-extension switch,
• Navigate to chrome://settings/content/pdfDocuments and verify that the Download PDF files... setting is set to ON and is disabled,
• In a new tab, navigate to a PDF URL (e.g. http://www.pdf995.com/samples/pdf.pdf),
• Verify that the PDF is not shown in Brave and is instead being downloaded.

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions
• Verify test plan is specified in PR before merging to source

[emerick]

LGTM

[bbondy]

looks good!

[mkarolin]

master: dab4abb
0.60.x: fc54d67
0.59.x: fad1fc2