add a human-readable explanation to allow the trace to show why the command buffer was synchronously flushed; This is handy to track down why we're experiencing long command buffer flushes on Android sometimes with GPU rasterization

BUG=

Review URL: https://codereview.chromium.org/132243002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247514 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247514