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..b8e094c8d847ff09fa44bb3b7e529b30f68096f3 100644
--- a/cc/layers/picture_layer_impl.cc
+++ b/cc/layers/picture_layer_impl.cc
@@ -790,27 +790,46 @@ void PictureLayerImpl::MarkVisibleResourcesAsRequired() const {
   // 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)
+  if (ShouldHaveLowResTiling()) {
+    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)
+        twin_high_res = GetTwinTiling(high_res);
+    }
+  } else {
+    if (high_res && tilings_->num_tilings() == 1 &&
+        twin_layer_ && twin_layer_->tilings_->num_tilings() == 1) {
       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 ||
-      twin_layer_->layer_tree_impl()->RequiresHighResToDraw() ||
-      draw_properties().screen_space_transform !=
-          twin_layer_->draw_properties().screen_space_transform) {
-    twin_high_res = NULL;
-    twin_low_res = NULL;
+
+  if (ShouldHaveLowResTiling()) {
+    if (!twin_high_res || !twin_low_res ||
+        twin_layer_->layer_tree_impl()->RequiresHighResToDraw() ||
+        draw_properties().screen_space_transform !=
+            twin_layer_->draw_properties().screen_space_transform) {
+      twin_high_res = NULL;
+      twin_low_res = NULL;
+    }
+  } else {
+    if (!twin_high_res ||
+        twin_layer_->layer_tree_impl()->RequiresHighResToDraw() ||
+        draw_properties().screen_space_transform !=
+            twin_layer_->draw_properties().screen_space_transform) {
+      twin_high_res = NULL;
+    }
   }
 
   // As a second pass, mark as required any visible high res tiles not filled in
   // by acceptable non-ideal tiles from the first pass.
   if (MarkVisibleTilesAsRequired(
-      high_res, twin_high_res, contents_scale_x(), rect, missing_region)) {
+      high_res, twin_high_res, contents_scale_x(), rect, missing_region) &&
+      ShouldHaveLowResTiling()) {
     // 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