Revert "overview: Fix crash with hiding title bars on entry."

This reverts commit b8738136.

Reason for revert: crbug.com/863795

Original change's description:
> 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}

TBR=oshima@chromium.org,sammiequon@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 863795, 843851
Change-Id: Ibc5965bc51a62e08b44d034a81f6954b8e799cad
Reviewed-on: https://chromium-review.googlesource.com/1142166Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576135}

ash/wm/overview/scoped_transform_overview_window.cc

@@ -279,8 +279,9 @@ bool ScopedTransformOverviewWindow::HideTitleBarAndAnimate(
   return true;
 }
 
-void ScopedTransformOverviewWindow::OnCompositingDidCommit(
-    ui::Compositor* compositor) {
+void ScopedTransformOverviewWindow::OnCompositingStarted(
+    ui::Compositor* compositor,
+    base::TimeTicks start_time) {
   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 {}