Fix flaky CopyOutRequest tests

SkiaCopyOutputScalingPixelTest.ScaledCopyOfDrawnFrame has been flaky
since https://crrev.com/c/2256225 landed. This change made it so that
CopyOutputResults were serialized on worker threads, which breaks the
ordering the test expects. Switch to using the test main thread to run
the result callbacks to remove data race on |dummy_ran|.

Bug: 1100421
Change-Id: Id19df859ed6af48cce85c16689122fa33e171184
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2274257
Commit-Queue: kylechar <kylechar@chromium.org>
Reviewed-by: weiliangc <weiliangc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#783960}

components/viz/service/display/copy_output_scaling_pixeltest.cc

@@ -9,6 +9,7 @@
 
 #include "base/bind.h"
 #include "base/run_loop.h"
+#include "base/threading/sequenced_task_runner_handle.h"
 #include "build/build_config.h"
 #include "cc/test/pixel_test.h"
 #include "cc/test/pixel_test_utils.h"
@@ -145,6 +146,8 @@ class CopyOutputScalingPixelTest
       // results of the main copy request (below) if the GL state is not
       // properly restored.
       request->SetUniformScaleRatio(1, 10);
+      // Ensure the result callback is run on test main thread.
+      request->set_result_task_runner(base::SequencedTaskRunnerHandle::Get());
       list.front()->copy_requests.push_back(std::move(request));
 
       // Add a copy request to the root RenderPass, to capture the results of
@@ -164,6 +167,8 @@ class CopyOutputScalingPixelTest
       request->set_result_selection(
           copy_output::ComputeResultRect(copy_rect, scale_from_, scale_to_));
       request->SetScaleRatio(scale_from_, scale_to_);
+      // Ensure the result callback is run on test main thread.
+      request->set_result_task_runner(base::SequencedTaskRunnerHandle::Get());
       list.back()->copy_requests.push_back(std::move(request));
 
       renderer()->DecideRenderPassAllocationsForFrame(list);