Make WebView::close not crash and start to fix navigation in SkyShell

sky/shell/ui/animator.cc

Index: sky/shell/ui/animator.cc
diff --git a/sky/shell/ui/animator.cc b/sky/shell/ui/animator.cc
index e7f299b97c99012a5404526bb52bdf3233ff049d..0833259c65d5740b58d9ad1105a20381aa484637 100644
--- a/sky/shell/ui/animator.cc
+++ b/sky/shell/ui/animator.cc
@@ -34,9 +34,17 @@ void Animator::RequestFrame() {
   }
 }
 
+void Animator::CancelFrameRequest() {
+  engine_requested_frame_ = false;
+}
+
 void Animator::BeginFrame() {
   DCHECK(frame_in_progress_);
-  DCHECK(engine_requested_frame_);
+  // There could be a request in the message loop at time of cancel.
+  if (!engine_requested_frame_) {
+    frame_in_progress_ = false;
+    return;
+  }
   engine_requested_frame_ = false;
 
   engine_->BeginFrame(base::TimeTicks::Now());