[Android] Speculative fix for crash during overscroll effect creation

Crash reports indicate that the WindowAndroid's Compositor may be NULL
during |RenderWidgetHostViewAndroid::SetContentViewCore()|. While this
is a somewhat surprising result, it is in theory possible and should
probably not be fatal. Avoid creating the overscroll effect in this
case, instead creating it after the compositor is (re-)attached.

BUG=407208

Review URL: https://codereview.chromium.org/503813003

Cr-Commit-Position: refs/heads/master@{#291835}

content/browser/renderer_host/render_widget_host_view_android.cc

@@ -1601,6 +1601,7 @@ void RenderWidgetHostViewAndroid::SetContentViewCore(
 
   bool resize = false;
   if (content_view_core != content_view_core_) {
+    overscroll_effect_.reset();
     selection_controller_.reset();
     ReleaseLocksOnSurface();
     resize = true;
@@ -1631,9 +1632,8 @@ void RenderWidgetHostViewAndroid::SetContentViewCore(
   if (!selection_controller_)
     selection_controller_.reset(new TouchSelectionController(this));
 
-  if (!content_view_core_)
-    overscroll_effect_.reset();
-  else if (overscroll_effect_enabled_ && !overscroll_effect_)
+  if (overscroll_effect_enabled_ && !overscroll_effect_ &&
+      content_view_core_->GetWindowAndroid()->GetCompositor())
     overscroll_effect_ = CreateOverscrollEffect(content_view_core_);
 }
 
@@ -1656,10 +1656,18 @@ void RenderWidgetHostViewAndroid::OnCompositingDidCommit() {
   RunAckCallbacks();
 }
 
+
+void RenderWidgetHostViewAndroid::OnAttachCompositor() {
+  DCHECK(content_view_core_);
+  if (overscroll_effect_enabled_ && !overscroll_effect_)
+    overscroll_effect_ = CreateOverscrollEffect(content_view_core_);
+}
+
 void RenderWidgetHostViewAndroid::OnDetachCompositor() {
   DCHECK(content_view_core_);
   DCHECK(!using_synchronous_compositor_);
   RunAckCallbacks();
+  overscroll_effect_.reset();
 }
 
 void RenderWidgetHostViewAndroid::OnVSync(base::TimeTicks frame_time,

content/browser/renderer_host/render_widget_host_view_android.h

@@ -199,7 +199,7 @@ class CONTENT_EXPORT RenderWidgetHostViewAndroid
 
   // ui::WindowAndroidObserver implementation.
   virtual void OnCompositingDidCommit() OVERRIDE;
-  virtual void OnAttachCompositor() OVERRIDE {}
+  virtual void OnAttachCompositor() OVERRIDE;
   virtual void OnDetachCompositor() OVERRIDE;
   virtual void OnVSync(base::TimeTicks frame_time,
                        base::TimeDelta vsync_period) OVERRIDE;