Add an about:flag to control video casting. The feature will be off by default.

BUG=298386

This reverts commit b70403fc1eb901266182d2f40b57849b60fe6716.

Review URL: https://codereview.chromium.org/24768003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225586 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225586