cc: Always use combined_bin when computing GPU memory manager stats.

By only including memory used for pending or active tree in GPU
memory manager stats we often report usage that will cause the
manager to think our requirements are lower than they are.
Always using the combined bin seem to produce better results.

BUG=342261

Review URL: https://codereview.chromium.org/153263005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250556 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250556