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This allows HTML5/WebRTC audio and video capture to work inside apps.

* Apps in app_shell are given access to the first available
 camera and microphone via WebContentsDelegate::
 ProcessMediaAccessRequest().
* Introduce media_capture_util so code can be shared 
 between ShellExtensionHostDelegate (for audio capture from 
 background pages) and ShellAppWindow (for normal windows).
* Extract ShellExtensionHostDelegate into its own file 
 because it's a little too big to sit where it is.

BUG=404299
TEST=manual, see app attached to bug

Review URL: https://codereview.chromium.org/493453004

Cr-Commit-Position: refs/heads/master@{#291534}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@291534 0039d316-1c4b-4281-b951-d872f2087c98