Mus: Fixes rendering of transparent containers.

components/mus/ws/platform_display.cc

Index: components/mus/ws/platform_display.cc
diff --git a/components/mus/ws/platform_display.cc b/components/mus/ws/platform_display.cc
index 46aa239127d72eb048503a6f184aa89bbebb9601..14095f10c83bfa520cac687628c48ea36bc644ee 100644
--- a/components/mus/ws/platform_display.cc
+++ b/components/mus/ws/platform_display.cc
@@ -65,7 +65,6 @@ void DrawWindowTree(cc::RenderPass* pass,
   const gfx::Rect absolute_bounds =
       window->bounds() + parent_to_root_origin_offset;
   std::vector<ServerWindow*> children(window->GetChildren());
-  // TODO(rjkroege, fsamuel): Make sure we're handling alpha correctly.
   const float combined_opacity = opacity * window->opacity();
   for (auto it = children.rbegin(); it != children.rend(); ++it) {
     DrawWindowTree(pass, *it, absolute_bounds.OffsetFromOrigin(),
@@ -94,7 +93,7 @@ void DrawWindowTree(cc::RenderPass* pass,
                 bounds_at_origin.size() /* layer_bounds */,
                 bounds_at_origin /* visible_layer_bounds */,
                 bounds_at_origin /* clip_rect */, false /* is_clipped */,
-                window->opacity(), SkXfermode::kSrcOver_Mode,
+                combined_opacity, SkXfermode::kSrcOver_Mode,
                 0 /* sorting-context_id */);
     auto quad = pass->CreateAndAppendDrawQuad<cc::SurfaceDrawQuad>();
     quad->SetAll(sqs, bounds_at_origin /* rect */,
@@ -115,7 +114,7 @@ void DrawWindowTree(cc::RenderPass* pass,
                 bounds_at_origin.size() /* layer_bounds */,
                 bounds_at_origin /* visible_layer_bounds */,
                 bounds_at_origin /* clip_rect */, false /* is_clipped */,
-                window->opacity(), SkXfermode::kSrcOver_Mode,
+                combined_opacity, SkXfermode::kSrcOver_Mode,
                 0 /* sorting-context_id */);
 
     auto quad = pass->CreateAndAppendDrawQuad<cc::SurfaceDrawQuad>();