Change the name of the generated pdf.sandbox.external for mozilla-central

This patch is blocking https://phabricator.services.mozilla.com/D148600.

Author: calixteman

gulpfile.js

@@ -397,7 +397,7 @@ function createSandboxExternal(defines) {
     transform("utf8", content => {
       content = preprocessor2.preprocessPDFJSCode(ctx, content);
       return `${licenseHeader}\n${content}`;
-    })
+    }).pipe(rename("pdf.sandbox.external.jsm"))
   );
 }