Fixing crash in PictureLayerImpl::MarkVisibleResourcesAsRequired when low-res tiles are disabled

cc/layers/picture_layer_impl.cc

Index: cc/layers/picture_layer_impl.cc
diff --git a/cc/layers/picture_layer_impl.cc b/cc/layers/picture_layer_impl.cc
index 85249812d5fbc9d5ea119537e014ae5e80eb80b2..13f95a9f5da73e23a580a67ee6f65b96c88eff73 100644
--- a/cc/layers/picture_layer_impl.cc
+++ b/cc/layers/picture_layer_impl.cc
@@ -787,19 +787,26 @@ void PictureLayerImpl::MarkVisibleResourcesAsRequired() const {
   const PictureLayerTiling* twin_low_res = NULL;
 
   // As a simplification, only allow activating to skip twin tiles that the
-  // active layer is also missing when both this layer and its twin have 2
-  // tilings (high and low).  This avoids having to iterate/track coverage of
-  // non-ideal tilings during the last draw call on the active layer.
-  if (high_res && low_res && tilings_->num_tilings() == 2 &&
-      twin_layer_ && twin_layer_->tilings_->num_tilings() == 2) {
-    twin_low_res = GetTwinTiling(low_res);
-    if (twin_low_res)
+  // active layer is also missing when both this layer and its twin have
+  // "simple" sets of tilings: only 2 tilings (high and low) for case, when low
+  // res is enabled; or only 1 high res tiling, when low res is disabled. This
+  // avoids having to iterate/track coverage of non-ideal tilings during the
+  // last draw call on the active layer.
+  bool should_have_low_res = ShouldHaveLowResTiling();
+  int num_tilings = tilings_->num_tilings();
+  if ((high_res && low_res && num_tilings == 2 ||
+       high_res && !should_have_low_res && num_tilings == 1) &&

[Sergey, 2014/05/16 02:26:13]

How about this one, did it get any better?... Or please see what it was in patch
set 1.

+      twin_layer_ && num_tilings == twin_layer_->tilings_->num_tilings()) {
+    if (should_have_low_res)
+      twin_low_res = GetTwinTiling(low_res);
+    if (twin_low_res || !should_have_low_res)
       twin_high_res = GetTwinTiling(high_res);
   }
+
   // If this layer and its twin have different transforms, then don't compare
   // them and only allow activating to high res tiles, since tiles on each layer
   // will be in different places on screen.
-  if (!twin_high_res || !twin_low_res ||
+  if (!twin_high_res || (!twin_low_res && should_have_low_res) ||
       twin_layer_->layer_tree_impl()->RequiresHighResToDraw() ||
       draw_properties().screen_space_transform !=
           twin_layer_->draw_properties().screen_space_transform) {
@@ -814,9 +821,11 @@ void PictureLayerImpl::MarkVisibleResourcesAsRequired() const {
     // As an optional third pass, if a high res tile was skipped because its
     // twin was also missing, then fall back to mark low res tiles as required
     // in case the active twin is substituting those for missing high res
-    // content.
-    MarkVisibleTilesAsRequired(
-        low_res, twin_low_res, contents_scale_x(), rect, missing_region);
+    // content. Only suitable, when low res is enabled.
+    if (should_have_low_res) {
+      MarkVisibleTilesAsRequired(
+          low_res, twin_low_res, contents_scale_x(), rect, missing_region);
+    }
   }
 }