GpuVideoDecoder: replace gvd_loop_proxy_ with a clearer single-threaded story.

Review URL: https://codereview.chromium.org/108203006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243033 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243033