Enable the embedded L1/EME support in WebView.

- Add ExternalVideoSurfaceContainer which handles the external surface for the hole punching in WebView.
- Refactor the callbacks of ContentViewClient on the hole-punching to make them have the single purpose per each function.

BUG=329447

Review URL: https://codereview.chromium.org/132233042

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252571 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252571