GpuVideoDecodeAccelerator: don't hold a lock while deleting self.

BUG=323594

Review URL: https://codereview.chromium.org/88253004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237530 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237530