cc : Initialize solid color scrollbar layer on main thread to 0.

cc/layers/solid_color_scrollbar_layer.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/layers/solid_color_scrollbar_layer.h"
 
 #include <memory>
 
 #include "cc/layers/layer_impl.h"
 #include "cc/layers/solid_color_scrollbar_layer_impl.h"
@@ skipping 28 matching lines @@
 SolidColorScrollbarLayer::SolidColorScrollbarLayer(
     ScrollbarOrientation orientation,
     int thumb_thickness,
     int track_start,
     bool is_left_side_vertical_scrollbar,
     int scroll_layer_id)
     : scroll_layer_id_(Layer::INVALID_ID),
       orientation_(orientation),
       thumb_thickness_(thumb_thickness),
       track_start_(track_start),
-      is_left_side_vertical_scrollbar_(is_left_side_vertical_scrollbar) {}
+      is_left_side_vertical_scrollbar_(is_left_side_vertical_scrollbar) {
+  Layer::SetOpacity(0.f);
+}
 
 SolidColorScrollbarLayer::~SolidColorScrollbarLayer() {}
 
 ScrollbarLayerInterface* SolidColorScrollbarLayer::ToScrollbarLayer() {
   return this;
 }
 
+void SolidColorScrollbarLayer::SetOpacity(float opacity) {
+  // The opacity of a solid color scrollbar layer is always 0 on main thread.
+  DCHECK_EQ(opacity, 0.f);

[jaydasika, 2016/05/24 23:37:23]

I am not sure if we should still be overriding this function, but it seems safe
to do it? Wdyt ?

+  Layer::SetOpacity(opacity);
+}
+
 void SolidColorScrollbarLayer::PushPropertiesTo(LayerImpl* layer) {
   Layer::PushPropertiesTo(layer);
   SolidColorScrollbarLayerImpl* scrollbar_layer =
       static_cast<SolidColorScrollbarLayerImpl*>(layer);
 
   scrollbar_layer->SetScrollLayerId(scroll_layer_id_);
 }
 
 void SolidColorScrollbarLayer::SetNeedsDisplayRect(const gfx::Rect& rect) {
   // Never needs repaint.
@@ skipping 50 matching lines @@
       proto.solid_scrollbar();
   scroll_layer_id_ = scrollbar.scroll_layer_id();
   thumb_thickness_ = scrollbar.thumb_thickness();
   track_start_ = scrollbar.track_start();
   is_left_side_vertical_scrollbar_ =
       scrollbar.is_left_side_vertical_scrollbar();
   orientation_ = ScrollbarOrientationFromProto(scrollbar.orientation());
 }
 
 }  // namespace cc