cc: Skip unused SharedQuadStates when pickling.

If we cull DrawQuads from the scene, we can be left with
SharedQuadStates that no DrawQuads are left actually using. It's a
waste of time/space to serialize these, so just skip them.

Tests:
CCMessagesTest.UnusedSharedQuadStates

BUG=311672
R=piman

Review URL: https://codereview.chromium.org/56153004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234800 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234800