cc: Add a resource_provider_ dcheck in resource_pool creation.

This patch just adds one DCHECK in order to help diagnose a crash that
seems to happen when we create a resource_pool with a null resource
provider.

R=enne
BUG=419086
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1243773003

Cr-Commit-Position: refs/heads/master@{#339747}

cc/trees/layer_tree_host_impl.cc

@@ -2093,6 +2093,9 @@ void LayerTreeHostImpl::CreateResourceAndTileTaskWorkerPool(
     scoped_ptr<ResourcePool>* resource_pool,
     scoped_ptr<ResourcePool>* staging_resource_pool) {
   DCHECK(GetTaskRunner());
+  // TODO(vmpstr): Make this a DCHECK (or remove) when crbug.com/419086 is
+  // resolved.
+  CHECK(resource_provider_);
 
   // Pass the single-threaded synchronous task graph runner to the worker pool
   // if we're in synchronous single-threaded mode.