Implement media cast buttons
    
This implements a Cast button in the default media controls, and a second,
overlayed Cast button on videos that don't use the default controls.
    
Pressing the Cast button asks the browser UI to bring up cast device
selection menu. If a device is selected the video or audio will be played
on that device, rather than locally.
    
The Cast buttons are only displayed when there are cast devices available.
Currently Blink will only ever be told that there are cast devices available
by Chrome for Android.
    
BUG=390125

Review URL: https://codereview.chromium.org/291163004

git-svn-id: svn://svn.chromium.org/blink/trunk@182077 bbb929c8-8fbe-4397-9dbb-9b2b20218538