[2/5] Disable transformed rasterization if layer is opaque

cc/layers/picture_layer.cc

Index: cc/layers/picture_layer.cc
diff --git a/cc/layers/picture_layer.cc b/cc/layers/picture_layer.cc
index 40742e50ea8b845178c2eeeb20a37e59219a473a..6cacd6fca499d11388277351b805970c77e541e6 100644
--- a/cc/layers/picture_layer.cc
+++ b/cc/layers/picture_layer.cc
@@ -248,6 +248,14 @@ bool PictureLayer::ShouldUseTransformedRasterization() const {
   if (!picture_layer_inputs_.allow_transformed_rasterization)
     return false;
 
+  // Background color overfill is undesirable with transformed rasterization.
+  // However, without background overfill, the tiles will be non-opaque on
+  // external edges, and layer opaque region can't be computed in layer space
+  // due to rounding under extreme scaling. This defeats many opaque layer
+  // optimization. Prefer optimization over quality for this particular case.
+  if (contents_opaque())
+    return false;
+
   const TransformTree& transform_tree =
       layer_tree_host()->property_trees()->transform_tree;
   DCHECK(!transform_tree.needs_update());