Set aura::Window transparency in SurfaceTreeHost::UpdateHostWindowBounds().

SurfaceTreeHost::UpdateHostWindowBounds() used to call
ui::Layer::SetFillsBoundsOpaquely() directly. With this CL, the method
is called through aura::Window::SetTransparent(). This is equivalent,
except that the |transparent_| bit of the aura::Window, on which
aura::WindowOcclusionTracker depends, is also updated.

This CL also adds a ScopedPauseOcclusionTracking instance to ensure
that occlusion isn't recomputed multiple times during a call to
SurfaceTreeHost::UpdateHostWindowBounds().

Bug: 668690
Change-Id: I4f6181d4e247f2a54d96569b3379964c9253246a
Reviewed-on: https://chromium-review.googlesource.com/970746Reviewed-by: David Reveman <reveman@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544362}

components/exo/surface_tree_host.cc

@@ -19,6 +19,7 @@
 #include "ui/aura/window.h"
 #include "ui/aura/window_delegate.h"
 #include "ui/aura/window_event_dispatcher.h"
+#include "ui/aura/window_occlusion_tracker.h"
 #include "ui/aura/window_targeter.h"
 #include "ui/aura/window_tree_host.h"
 #include "ui/base/cursor/cursor.h"
@@ -105,6 +106,11 @@ void SurfaceTreeHost::SetRootSurface(Surface* root_surface) {
   if (root_surface == root_surface_)
     return;
 
+  // This method applies multiple changes to the window tree. Use
+  // ScopedPauseOcclusionTracking to ensure that occlusion isn't recomputed
+  // before all changes have been applied.
+  aura::WindowOcclusionTracker::ScopedPauseOcclusionTracking pause_occlusion;
+
   if (root_surface_) {
     root_surface_->window()->Hide();
     host_window_->RemoveChild(root_surface_->window());
@@ -320,12 +326,18 @@ void SurfaceTreeHost::SubmitCompositorFrame() {
 // SurfaceTreeHost, private:
 
 void SurfaceTreeHost::UpdateHostWindowBounds() {
+  // This method applies multiple changes to the window tree. Use
+  // ScopedPauseOcclusionTracking to ensure that occlusion isn't recomputed
+  // before all changes have been applied.
+  aura::WindowOcclusionTracker::ScopedPauseOcclusionTracking pause_occlusion;
+
   gfx::Rect bounds = root_surface_->surface_hierarchy_content_bounds();
   host_window_->SetBounds(
       gfx::Rect(host_window_->bounds().origin(), bounds.size()));
-  host_window_->layer()->SetFillsBoundsOpaquely(
+  const bool fills_bounds_opaquely =
       bounds.size() == root_surface_->content_size() &&
-      root_surface_->FillsBoundsOpaquely());
+      root_surface_->FillsBoundsOpaquely();
+  host_window_->SetTransparent(!fills_bounds_opaquely);
 
   root_surface_origin_ = gfx::Point() - bounds.OffsetFromOrigin();
   root_surface_->window()->SetBounds(gfx::Rect(