Replace MessageLoopProxy with SingleThreadTaskRunner for media/base/.

BUG=315922
R=xhwang@chromium.org

Review URL: https://codereview.chromium.org/64033005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242180 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242180
Convert clang=1 to the GN build.

This maps clang=1 in GYP to is_clang=true in GN.

BUG=
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/108513014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242181 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242181