Refactor WebMediaPlayerImpl's multithreaded painting code into VideoFramePainter.

BUG=335345

Review URL: https://codereview.chromium.org/130423005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247010 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247010