GestureNav: Fix a bug in OverlaySceneLayer destruction

The crash at OverlaySceneLayer destructor comes apparently from
ui::WindowAndroid being in the destroyed state while its
::RemoveObserver method is called. It makes OverlaySceneLayer
destructor not the appropriate place to call it, since the
destruction order of the objects are not clear.

This CL changes the place where WindowAndroidObserver is added
or deleted i.e. when WindowAndroid's compositor is attached
or detached. WindowAndroid is guaranteed to be available
when these observer event is invoked, so it's safe to do the
operation.

Bug: 1120832
Change-Id: Ib43c420f4d5af51fca4377e92d46a408d41e86f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2371384Reviewed-by: Matthew Jones <mdjones@chromium.org>
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801125}

chrome/browser/android/compositor/scene_layer/overscroll_scene_layer.cc

@@ -18,12 +18,11 @@ OverscrollSceneLayer::OverscrollSceneLayer(JNIEnv* env,
     : SceneLayer(env, jobj),
       window_(ui::WindowAndroid::FromJavaWindowAndroid(jwindow)),
       glow_effect_(std::make_unique<ui::OverscrollGlow>(this)) {
-  window_->AddObserver(this);
+  if (window_->GetCompositor())
+    window_->AddObserver(this);
 }
 
-OverscrollSceneLayer::~OverscrollSceneLayer() {
-  window_->RemoveObserver(this);
-}
+OverscrollSceneLayer::~OverscrollSceneLayer() {}
 
 std::unique_ptr<ui::EdgeEffect> OverscrollSceneLayer::CreateEdgeEffect() {
   return std::make_unique<ui::EdgeEffect>(resource_manager_);
@@ -72,6 +71,14 @@ void OverscrollSceneLayer::OnAnimate(base::TimeTicks frame_time) {
     window_->SetNeedsAnimate();
 }
 
+void OverscrollSceneLayer::OnAttachCompositor() {
+  window_->AddObserver(this);
+}
+
+void OverscrollSceneLayer::OnDetachCompositor() {
+  window_->RemoveObserver(this);
+}
+
 void OverscrollSceneLayer::SetContentTree(
     JNIEnv* env,
     const JavaParamRef<jobject>& jobj,

chrome/browser/android/compositor/scene_layer/overscroll_scene_layer.h

@@ -52,8 +52,8 @@ class OverscrollSceneLayer : public SceneLayer,
   // ui::WindowAndroidObserver implementation.
   void OnCompositingDidCommit() override {}
   void OnRootWindowVisibilityChanged(bool visible) override {}
-  void OnAttachCompositor() override {}
-  void OnDetachCompositor() override {}
+  void OnAttachCompositor() override;
+  void OnDetachCompositor() override;
   void OnAnimate(base::TimeTicks begin_frame_time) override;
   void OnActivityStopped() override {}
   void OnActivityStarted() override {}