[HBD] Only use Plugin Content Settings for Flash.

All other plugins (PDF, Widevine, Nacl), as well as custom plugins,
should be treated as JavaScript.

This is because starting in 55, all the Plugin content settings UI
surfaces have been renamed the Flash content settings.

This is related to the chrome://plugins deprecation.

BUG=615738

Review-Url: https://codereview.chromium.org/2356053002
Cr-Commit-Position: refs/heads/master@{#421348}
15 files changed