Allow to have a transparent UA dependent background.

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 7688b974e54bb6138c43f2081a4f2c1e6d759b14..5d1374814d078bb1b607f29ea0963952c9b4958a 100644
--- a/cc/layers/picture_layer_impl.cc
+++ b/cc/layers/picture_layer_impl.cc
@@ -325,8 +325,19 @@ void PictureLayerImpl::AppendQuads(RenderPass* render_pass,
         case TileDrawInfo::SOLID_COLOR_MODE: {
           SolidColorDrawQuad* quad =
               render_pass->CreateAndAppendDrawQuad<SolidColorDrawQuad>();
-          quad->SetNew(shared_quad_state, geometry_rect, visible_geometry_rect,
-                       draw_info.solid_color(), false);
+
+          // At the beginning of the for loop above, opaque_rect is defined
+          // against contents_opaque(). If it was actually the case then use it
+          // otherwise use quad->SetNew which configure opaque_rect on other
+          // condition.
+          if (contents_opaque()) {
+            quad->SetAll(shared_quad_state, geometry_rect, opaque_rect,
+                         visible_geometry_rect, false, draw_info.solid_color(),
+                         false);
+          } else {
+            quad->SetNew(shared_quad_state, geometry_rect,
+                         visible_geometry_rect, draw_info.solid_color(), false);
+          }

[trchen, 2016/03/08 01:57:21]

Why is this change needed? This only changes the behavior when a layer is
opaque, but what the thing you want to change is how non-opaque pages being
handled.

           ValidateQuadResources(quad);
           iter->draw_info().set_was_ever_used_to_draw();
           has_draw_quad = true;