Do not call LayerAnimator::SetOpacity if the new value is same as the target value.

I think this is safer to merge than https://codereview.chromium.org/2291923002/. I'll remove the check once the bug 64223 is fixed.

BUG=642222

Review-Url: https://codereview.chromium.org/2291933002
Cr-Commit-Position: refs/heads/master@{#415341}

components/exo/shell_surface.cc

@@ -1253,6 +1253,7 @@ void ShellSurface::UpdateShadow() {
       window->StackChildAtBottom(shadow_underlay_);
     }
 
+    float shadow_underlay_opacity = rectangular_shadow_background_opacity_;
     // Put the black background layer behind the window if
     // 1) the window is in immersive fullscreen.
     // 2) the window can control the bounds of the window in fullscreen (
@@ -1264,14 +1265,20 @@ void ShellSurface::UpdateShadow() {
         window->layer()->transform().IsIdentity()) {
       gfx::Point origin;
       origin -= window->bounds().origin().OffsetFromOrigin();
-      gfx::Rect background_bounds(origin, window->parent()->bounds().size());
-      shadow_underlay_->SetBounds(background_bounds);
-      shadow_underlay_->layer()->SetOpacity(1.f);
-    } else {
-      shadow_underlay_->SetBounds(shadow_bounds);
-      shadow_underlay_->layer()->SetOpacity(
-          rectangular_shadow_background_opacity_);
+      shadow_bounds.set_origin(origin);
+      shadow_bounds.set_size(window->parent()->bounds().size());
+      shadow_underlay_opacity = 1.0f;
     }
+
+    shadow_underlay_->SetBounds(shadow_bounds);
+
+    // TODO(oshima): Setting to the same value should be no-op.
+    // crbug.com/642223.
+    if (shadow_underlay_opacity !=
+        shadow_underlay_->layer()->GetTargetOpacity()) {
+      shadow_underlay_->layer()->SetOpacity(shadow_underlay_opacity);
+    }
+
     shadow_underlay_->Show();
 
     wm::Shadow* shadow = wm::ShadowController::GetShadowForWindow(window);