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

cc/layers/solid_color_scrollbar_layer.cc

Index: cc/layers/solid_color_scrollbar_layer.cc
diff --git a/cc/layers/solid_color_scrollbar_layer.cc b/cc/layers/solid_color_scrollbar_layer.cc
index 131e7735cb194a310363397636227267125f9cb7..9e4c65fd6512ca0085494aa851ce08c66cb31b6e 100644
--- a/cc/layers/solid_color_scrollbar_layer.cc
+++ b/cc/layers/solid_color_scrollbar_layer.cc
@@ -46,7 +46,9 @@ SolidColorScrollbarLayer::SolidColorScrollbarLayer(
       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() {}
 
@@ -54,6 +56,12 @@ 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 =