cc: Add null checks for GrContext created by ContextProviderCommandBuffer.

cc/resources/scoped_gpu_raster.cc

Index: cc/resources/scoped_gpu_raster.cc
diff --git a/cc/resources/scoped_gpu_raster.cc b/cc/resources/scoped_gpu_raster.cc
index 2f5c5c9c3426a42eca7532cb8919611884a983d1..b816d1de3d19ce716b1561c57be212ba5bf9b99d 100644
--- a/cc/resources/scoped_gpu_raster.cc
+++ b/cc/resources/scoped_gpu_raster.cc
@@ -30,12 +30,14 @@ void ScopedGpuRaster::BeginGpuRaster() {
   gl->PushGroupMarkerEXT(0, "GpuRasterization");
 
   class GrContext* gr_context = context_provider_->GrContext();
-  gr_context->resetContext();
+  if (gr_context)
+    gr_context->resetContext();
 }
 
 void ScopedGpuRaster::EndGpuRaster() {
   class GrContext* gr_context = context_provider_->GrContext();
-  gr_context->flush();
+  if (gr_context)
+    gr_context->flush();
 
   GLES2Interface* gl = context_provider_->ContextGL();