Revert of Remove CompositorObserver::OnCompositingEnded()

ui/compositor/test/draw_waiter_for_test.cc

Index: ui/compositor/test/draw_waiter_for_test.cc
diff --git a/ui/compositor/test/draw_waiter_for_test.cc b/ui/compositor/test/draw_waiter_for_test.cc
index e92e1a1a85d6eaa7408dcc1c5d60254d926d467a..cd2e5130abc9b95bef6f8b3113eb3c97a959b431 100644
--- a/ui/compositor/test/draw_waiter_for_test.cc
+++ b/ui/compositor/test/draw_waiter_for_test.cc
@@ -14,6 +14,11 @@ void DrawWaiterForTest::WaitForCompositingStarted(Compositor* compositor) {
   waiter.WaitImpl(compositor);
 }
 
+void DrawWaiterForTest::WaitForCompositingEnded(Compositor* compositor) {
+  DrawWaiterForTest waiter(WAIT_FOR_COMPOSITING_ENDED);
+  waiter.WaitImpl(compositor);
+}
+
 // static
 void DrawWaiterForTest::WaitForCommit(Compositor* compositor) {
   DrawWaiterForTest waiter(WAIT_FOR_COMMIT);
@@ -44,6 +49,11 @@ void DrawWaiterForTest::OnCompositingStarted(Compositor* compositor,
     wait_run_loop_->Quit();
 }
 
+void DrawWaiterForTest::OnCompositingEnded(Compositor* compositor) {
+  if (wait_event_ == WAIT_FOR_COMPOSITING_ENDED)
+    wait_run_loop_->Quit();
+}
+
 void DrawWaiterForTest::OnCompositingLockStateChanged(Compositor* compositor) {}
 
 void DrawWaiterForTest::OnCompositingShuttingDown(Compositor* compositor) {}