Added a CHECK to help track down where a nullptr is coming from.

Bug: 1077645
Change-Id: Iabcd01be3f3899a80ad861e6389077c54de2e0e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2232776
Commit-Queue: Frank Liberato <liberato@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Auto-Submit: Frank Liberato <liberato@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#775805}

media/gpu/windows/d3d11_texture_wrapper.cc

@@ -74,6 +74,9 @@ bool DefaultTexture2DWrapper::ProcessTexture(
   if (received_error_)
     return false;
 
+  // Temporary check to track down https://crbug.com/1077645
+  CHECK(texture);
+
   // It's okay to post and forget this call, since it'll be ordered correctly
   // with respect to any access on the gpu main thread.
   gpu_resources_.Post(FROM_HERE, &GpuResources::PushNewTexture,