cc: Don't clear transparent piles to an opaque color

cc/layers/layer.cc

Index: cc/layers/layer.cc
diff --git a/cc/layers/layer.cc b/cc/layers/layer.cc
index 8826b669b9f4b2d007e0c6fe52686e91ded14047..a3f16ed688ccd30a8cf46aa61467b7cbd51b6765 100644
--- a/cc/layers/layer.cc
+++ b/cc/layers/layer.cc
@@ -343,6 +343,25 @@ void Layer::SetBackgroundColor(SkColor background_color) {
   SetNeedsCommit();
 }
 
+SkColor Layer::SafeOpaqueBackgroundColor() const {
+  SkColor color = background_color();
+  if (SkColorGetA(color) == 255 && !contents_opaque()) {
+    color = SK_ColorTRANSPARENT;
+  } else if (SkColorGetA(color) != 255 && contents_opaque()) {
+    for (const Layer* layer = parent(); layer;
+         layer = layer->parent()) {
+      color = layer->background_color();
+      if (SkColorGetA(color) == 255)
+        break;
+    }
+    if (SkColorGetA(color) != 255)
+      color = layer_tree_host_->background_color();
+    if (SkColorGetA(color) != 255)
+      color = SkColorSetA(color, 255);
+  }
+  return color;
+}
+
 void Layer::CalculateContentsScale(
     float ideal_contents_scale,
     float device_scale_factor,