<webview>: Implement CopyFromCompositingSurface() for guests in <webview>

BUG=326756
R=alexst@chromium.org, fsamuel@chromium.org, kenrb@chromium.org

Review URL: https://codereview.chromium.org/99143004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240020 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240020