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

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 72a878b9de98ec2b4a201e871039dd33357d187d..b497601b69039177d5a0d0a6ebbf2d7af09653cf 100644
--- a/cc/layers/picture_layer_impl.cc
+++ b/cc/layers/picture_layer_impl.cc
@@ -216,23 +216,7 @@ void PictureLayerImpl::AppendQuads(QuadSink* quad_sink,
         if (quad_sink->Append(quad.PassAs<DrawQuad>(), append_quads_data))
           append_quads_data->num_missing_tiles++;
       } else {
-        SkColor color = background_color();
-        // TODO(wangxianzhu): Change the next |if| condition once we support
-        // finer-grain opaqueness. Ensure with the following DCHECK.
-        DCHECK(contents_opaque() || VisibleContentOpaqueRegion().IsEmpty());
-        if (SkColorGetA(color) != 255 && contents_opaque()) {
-          // If content is opaque, the occlusion tracker expects this layer to
-          // cover the background, so needs an opaque color.
-          for (const LayerImpl* layer = parent(); layer;
-               layer = layer->parent()) {
-            color = layer->background_color();
-            if (SkColorGetA(color) == 255)
-              break;
-          }
-          if (SkColorGetA(color) != 255)
-            color = layer_tree_impl()->background_color();
-          DCHECK_EQ(SkColorGetA(color), 255u);
-        }
+        SkColor color = SafeOpaqueBackgroundColor();
         scoped_ptr<SolidColorDrawQuad> quad = SolidColorDrawQuad::Create();
         quad->SetNew(shared_quad_state, geometry_rect, color, false);
         if (quad_sink->Append(quad.PassAs<DrawQuad>(), append_quads_data))