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

cc/layers/tiled_layer_impl.cc

@@ -54,6 +54,12 @@ TiledLayerImpl::~TiledLayerImpl() {
 ResourceProvider::ResourceId TiledLayerImpl::ContentsResourceId() const {
   // This function is only valid for single texture layers, e.g. masks.
   DCHECK(tiler_);
+  // It's possible the mask layer is created but has no size or otherwise
+  // can't draw.
+  if (tiler_->num_tiles_x() == 0 || tiler_->num_tiles_y() == 0)
+    return 0;
+
+  // Any other number of tiles other than 0 or 1 is incorrect for masks.
   DCHECK_EQ(tiler_->num_tiles_x(), 1);
   DCHECK_EQ(tiler_->num_tiles_y(), 1);