cc: Add null checks for GrContext created by ContextProviderCommandBuffer.

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index 0bf40594b49d519a242d450b1859190577331892..986ddaf0a3eaa990bf9c48ede51359992ce4c5a8 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -2166,6 +2166,16 @@ bool LayerTreeHostImpl::InitializeRenderer(
     return false;
   }
 
+  if (use_gpu_rasterization_) {

[danakj, 2015/05/12 23:40:43]

This can be changed by CreateAndSetRenderer below and suddenly become true I
think (I was hoping it was a one-way trip from true to false but the code
doesn't look like that to me.)

But maybe you can make UpdateGpuRasterizationStatus() before this. It's using
renderercaps but only as a proxy for the contextprovider caps. It's also
recreating layer resources and stuff which would maybe need to be split into
another function to run at the time when it currently is.

[vmiura, 2015/05/13 02:53:00]

Thanks for spotting this.

Hmm, I think it could also flip from false to true in
UpdateGpuRasterizationStatus()
via CommitComplete().

This doesn't look good to me anymore; we can handle failure in
InitializeRenderer() but after that I think the code assumes that
UpdateGpuRasterizationStatus() succeeds.

I'm thinking it's safer to back to "ifs" in the several places where GrContext()
is used, with big TODOs to remove them when GrContext can handle failure.

[piman, 2015/05/13 03:28:36]

We can simply rely on settings_.gpu_rasterization_enabled instead. If we /may/
use gpu rasterization, we should check the GrContext. Otherwise, we don't care.

[Stephen White, 2015/05/13 10:10:13]

In that case, you should probably check settings_.gpu_rasterization_forced as
well.

+    // Ensure we have a GrContext for GPU rasterization.
+    ContextProvider* context_provider =
+        output_surface->worker_context_provider();
+    if (!context_provider)

[piman, 2015/05/12 23:33:50]

nit: you can probably DCHECK this.

+      return false;
+    if (!context_provider->GrContext())
+      return false;
+  }
+
   output_surface_ = output_surface.Pass();
   resource_provider_ = ResourceProvider::Create(
       output_surface_.get(), shared_bitmap_manager_, gpu_memory_buffer_manager_,