Make the background of the cast button semi-transparent

The overlay cast button had a semi-transparent CSS background color.
Unfortunately the button image is treated in Blink as a background
image, so this modified the image, rather than simply providing
a background for it. As a result the cast button came out as grey
on grey on pale backgrounds.

Fix this by making the CSS background color transparent, and
modifying the icon image itself to make its background semi-transparent.

Since the CSS is in Blink the CSS change is in the Blink CL
https://codereview.chromium.org/658353002

BUG=424180

Review URL: https://codereview.chromium.org/661833005

Cr-Commit-Position: refs/heads/master@{#300130}

Author: aberent