Fix two crashes when resuming apps

@abarth

sky/shell/gpu/rasterizer.cc

@@ -71,10 +71,15 @@ void Rasterizer::DrawPicture(SkPicture* picture) {
 }
 
 void Rasterizer::OnOutputSurfaceDestroyed() {
-  CHECK(context_->MakeCurrent(surface_.get()));
-  ganesh_surface_.reset();
-  ganesh_context_.reset();
-  context_ = nullptr;
+  if (context_) {
+    CHECK(context_->MakeCurrent(surface_.get()));
+    ganesh_surface_.reset();
+    ganesh_context_.reset();
+    context_ = nullptr;
+  }
+  CHECK(!ganesh_surface_);
+  CHECK(!ganesh_context_);
+  CHECK(!context_);
   surface_ = nullptr;
 }
 

sky/shell/ui/animator.cc

@@ -48,7 +48,6 @@ void Animator::RequestFrame() {
 
 void Animator::Stop() {
   paused_ = true;
-  engine_requested_frame_ = false;
 }
 
 void Animator::Start() {
@@ -59,11 +58,16 @@ void Animator::Start() {
 void Animator::BeginFrame(int64_t time_stamp) {
   TRACE_EVENT_ASYNC_END0("sky", "Frame request pending", this);
   DCHECK(engine_requested_frame_);
-  engine_requested_frame_ = false;
-
   DCHECK(outstanding_requests_ > 0);
   DCHECK(outstanding_requests_ <= kPipelineDepth) << outstanding_requests_;
 
+  engine_requested_frame_ = false;
+
+  if (paused_) {
+    OnFrameComplete();
+    return;
+  }
+
   base::TimeTicks frame_time = time_stamp ?
       base::TimeTicks::FromInternalValue(time_stamp) : base::TimeTicks::Now();