overview: Fix crash with hiding title bars on entry.

Using OnCompositorStarted does not work for overview anymore. See stack
trace in bug.

Test: manual
Bug: 863795, 843851
Change-Id: I3dc8965cbece03ac36f1dc404c9b19b7dcac5b5e
Reviewed-on: https://chromium-review.googlesource.com/1138737
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575447}

ash/wm/overview/scoped_transform_overview_window.cc

@@ -278,9 +278,8 @@ bool ScopedTransformOverviewWindow::HideTitleBarAndAnimate(
   return true;
 }
 
-void ScopedTransformOverviewWindow::OnCompositingStarted(
-    ui::Compositor* compositor,
-    base::TimeTicks start_time) {
+void ScopedTransformOverviewWindow::OnCompositingDidCommit(
+    ui::Compositor* compositor) {
   views::Widget* widget = views::Widget::GetWidgetForNativeWindow(window_);
   DCHECK(widget);
   DCHECK_EQ(compositor, widget->GetCompositor());

ash/wm/overview/scoped_transform_overview_window.h

@@ -200,9 +200,9 @@ class ASH_EXPORT ScopedTransformOverviewWindow
   void OnImplicitAnimationsCompleted() override;
 
   // ui::CompositorObserver:
-  void OnCompositingDidCommit(ui::Compositor* compositor) override {}
+  void OnCompositingDidCommit(ui::Compositor* compositor) override;
   void OnCompositingStarted(ui::Compositor* compositor,
-                            base::TimeTicks start_time) override;
+                            base::TimeTicks start_time) override {}
   void OnCompositingEnded(ui::Compositor* compositor) override {}
   void OnCompositingLockStateChanged(ui::Compositor* compositor) override {}
   void OnCompositingChildResizing(ui::Compositor* compositor) override {}