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This patch adds the menu and the list of casts. The menu just shows the menu, casting feature is not implemented.

All changes are effective only when the command line flag is specified. And the resource (cast icons) may change later.

Major changes:
- Detect Cast extension and load it
- Discover casts and show them on the menu

BUG=305511
TEST=manually testted

Review URL: https://codereview.chromium.org/381073003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@282609 0039d316-1c4b-4281-b951-d872f2087c98