Fix corner case if another rotation request comes before the copy request finishes.

ash/rotator/screen_rotation_animator.cc

Index: ash/rotator/screen_rotation_animator.cc
diff --git a/ash/rotator/screen_rotation_animator.cc b/ash/rotator/screen_rotation_animator.cc
index a6eaccf6b435d42de2a595ddadf1823a29408325..b3bacdf3bbeffa3bcc5d1355efee2669ca879eae 100644
--- a/ash/rotator/screen_rotation_animator.cc
+++ b/ash/rotator/screen_rotation_animator.cc
@@ -421,6 +421,11 @@ void ScreenRotationAnimator::set_disable_animation_timers_for_test(
 }
 
 void ScreenRotationAnimator::StopAnimating() {
+  // |old_layer_tree_owner_| could be nullptr if another rotation request comes
+  // before the copy request finishes. At this time there is no animation.
+  if (!old_layer_tree_owner_)

[oshima, 2017/04/10 18:38:46]

what's the exact call sequence that reaches this condition?

My suggestion is that instead of having simple "bool is_rotating",
define state enum (e.g INIT, COPY_REQUESTED, ROTATING, DONE), and track the
state and do the right thing based
on the state.

[wutao, 2017/04/10 21:19:22]

Add the enum state, please review.

Current sequence is:
1) rotation_request when idle -> rotate -> finish.
2) rotation_request when rotating -> finish current animation to target position
and continue new rotation_request.

When we adding the copy request, the sequence becomes:
1) rotation_request when idle -> send copy request -> wait the copy result ->
rotate -> finish.
2) rotation_request when rotating -> finish current animation to target position
and continue new rotation_request.
3) rotation_request when wait the copy result -> I think we should ignore the
rotation request during this time. Otherwise, we need cancel the copy request
and abort current rotation. This complicates the situation and gain little from
it.

+    return;
+
   aura::Window* root_window = GetRootWindow(display_id_);
   for (ui::Layer* child_layer : root_window->layer()->children()) {
     if (child_layer == old_layer_tree_owner_->root())