cc: Remove TiledLayer mask DCHECK for 0 tiles

It seems that this is firing in practice, so don't DCHECK it.  Instead,
just return no resource at all since that mask layer can't draw.  This
will behave identically to the case where there is no mask at all.

BUG=165723

Review URL: https://codereview.chromium.org/136143004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245365 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245365