cc: Fix DCHECK when impl thread changes scale on a layer being hidden.

cc/resources/picture_layer_tiling_set.cc

Index: cc/resources/picture_layer_tiling_set.cc
diff --git a/cc/resources/picture_layer_tiling_set.cc b/cc/resources/picture_layer_tiling_set.cc
index e1659be78f592296b1a3bd78f4d32592588762df..7225d29c37621caab9bb718f4c39e675837054a1 100644
--- a/cc/resources/picture_layer_tiling_set.cc
+++ b/cc/resources/picture_layer_tiling_set.cc
@@ -65,6 +65,14 @@ void PictureLayerTilingSet::UpdateTilingsToCurrentRasterSource(
   // Copy over tilings that are shared with the |twin_set| tiling set (if it
   // exists).
   if (twin_set) {
+    if (twin_set->tilings_.empty()) {
+      // If the twin (pending) tiling set is empty, for example it does not have

[enne (OOO), 2015/02/14 00:31:05]

Are there any reasons why the active tree could still have valid content to be
showing but the pending twin has empty tilings?

[danakj, 2015/02/14 00:41:25]

We don't know what properties to use for the tilings. The tilings that are
currently on the active tree would be from then the last draw props happened
there. We have no valid live tiles rect or resolution to apply to those tilings
on activate.

I think you can construct a thing where the pending tree layer is not visible
but an animation will make it become visible at the same time you activate (a
race). Then the non-ideal tiling could in theory become useful there, if it
happened to be showing the same scale/position as the last active frame.

[enne (OOO), 2015/02/14 00:50:30]

I guess I'm trying to say that I don't understand why you know that's the case
just because the pending set is empty.  Why don't you know?  I feel like I'm
missing something.

[danakj, 2015/02/14 00:52:20]

The pending set is empty means UpdateTiles didn't happen on the pending layer.
Cuz UpdateTiles always makes a high res tiling. I can say this in the comment if
you like.

[enne (OOO), 2015/02/14 00:53:14]

Sure, but why does UpdateTiles not happening on the pending layer imply that
it's ok to remove everything from the active layer?

[danakj, 2015/02/14 00:57:38]

Ah. Well, the other choice is to leave behind a bunch of non-ideal tilings which
violates our expectations. We don't know which tiling would be high res.

When we activate, there was nothing visible in the pending tree that was part of
the requirements for activation. So there should not be anything on the active
tree that matters for the frame either.

+      // valid draw properties so no tilings, then we drop all tilings here as
+      // well, instead of leaving behind unshared tilings that are all
+      // non-ideal.
+      RemoveAllTilings();
+    }
+
     for (PictureLayerTiling* twin_tiling : twin_set->tilings_) {
       float contents_scale = twin_tiling->contents_scale();
       DCHECK_GE(contents_scale, minimum_contents_scale);