SkyShell DCHECKs on startup sometimes

sky/shell/gpu/rasterizer.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "sky/shell/gpu/rasterizer.h"
 
 #include "sky/shell/gpu/ganesh_context.h"
 #include "sky/shell/gpu/ganesh_surface.h"
 #include "third_party/skia/include/core/SkCanvas.h"
 #include "third_party/skia/include/core/SkPicture.h"
@@ skipping 26 matching lines @@
 
 void Rasterizer::OnAcceleratedWidgetAvailable(gfx::AcceleratedWidget widget) {
   surface_ = gfx::GLSurface::CreateViewGLSurface(widget);
   CHECK(surface_) << "GLSurface required.";
 }
 
 void Rasterizer::Draw(skia::RefPtr<SkPicture> picture) {
   if (!surface_)
     return;
 
+  gfx::Size size = GetSize(picture.get());
+  if (size.IsEmpty())
+    return;
+
   EnsureGLContext();
   CHECK(context_->MakeCurrent(surface_.get()));
-  EnsureGaneshSurface(GetSize(picture.get()));
+  EnsureGaneshSurface(size);
 
   SkCanvas* canvas = ganesh_surface_->canvas();
   canvas->drawPicture(picture.get());
   canvas->flush();
 
   surface_->SwapBuffers();
 }
 
 void Rasterizer::OnOutputSurfaceDestroyed() {
   ganesh_surface_.reset();
@@ skipping 12 matching lines @@
   ganesh_context_.reset(new GaneshContext(context_.get()));
 }
 
 void Rasterizer::EnsureGaneshSurface(const gfx::Size& size) {
   if (!ganesh_surface_ || ganesh_surface_->size() != size)
     ganesh_surface_.reset(new GaneshSurface(ganesh_context_.get(), size));
 }
 
 }  // namespace shell
 }  // namespace sky