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..c4ee9360f7b7848948a152a065a287926647cc39 100644
--- a/cc/layers/picture_layer_impl.cc
+++ b/cc/layers/picture_layer_impl.cc
@@ -787,19 +787,25 @@ 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();
+  if (high_res && (low_res || !should_have_low_res) && twin_layer_ &&
+      tilings_->num_tilings() == twin_layer_->tilings_->num_tilings() &&
+      tilings_->num_tilings() <= 2) {
+    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 +820,11 @@ 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);
+    }
   }
 }