Add checks to find source of null requests

Somehow ReadPixelsContext::request is null once it reaches
OnRGBAReadbackDone() done. Add some checks to figure out how that can
happen.

Bug: 1048251
Change-Id: If74854b5b332b1af347647c4c1c234e995afa944
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2037846Reviewed-by: Jonathan Backer <backer@chromium.org>
Commit-Queue: kylechar <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738563}

components/viz/service/display_embedder/skia_output_surface_impl.cc

@@ -596,6 +596,8 @@ void SkiaOutputSurfaceImpl::CopyOutput(
     const gfx::ColorSpace& color_space,
     std::unique_ptr<CopyOutputRequest> request) {
   DCHECK_CALLED_ON_VALID_THREAD(thread_checker_);
+  // TODO(crbug.com/1048251): Remove after finding source of null requests.
+  CHECK(request);
   if (!request->has_result_task_runner())
     request->set_result_task_runner(base::ThreadTaskRunnerHandle::Get());
 

components/viz/service/display_embedder/skia_output_surface_impl_on_gpu.cc

@@ -119,7 +119,10 @@ struct ReadPixelsContext {
       : request(std::move(request)),
         result_rect(result_rect),
         color_space(color_space),
-        impl_on_gpu(impl_on_gpu) {}
+        impl_on_gpu(impl_on_gpu) {
+    // TODO(crbug.com/1048251): Remove after finding source of null requests.
+    CHECK(this->request);
+  }
 
   std::unique_ptr<CopyOutputRequest> request;
   gfx::Rect result_rect;
@@ -255,6 +258,9 @@ void OnRGBAReadbackDone(
     return;
   }
 
+  // TODO(crbug.com/1048251): Remove after finding source of null requests.
+  CHECK(context->request);
+
   DCHECK_EQ(1, async_result->count());
 
   auto info = SkImageInfo::MakeN32Premul(context->result_rect.width(),
@@ -1140,6 +1146,9 @@ void SkiaOutputSurfaceImplOnGpu::CopyOutput(
   // TODO(crbug.com/898595): Do this on the GPU instead of CPU with Vulkan.
   DCHECK_CALLED_ON_VALID_THREAD(thread_checker_);
 
+  // TODO(crbug.com/1048251): Remove after finding source of null requests.
+  CHECK(request);
+
   // Clear |destroy_after_swap_| if we CopyOutput without SwapBuffers.
   base::ScopedClosureRunner cleanup(
       base::BindOnce([](std::vector<std::unique_ptr<SkDeferredDisplayList>>) {},