gpu: Mark overlay trace test as flaky

OverlayModeTraceTest_DirectComposition_Underlay is failing intermittenly
on nvidia-quadro-p400-win10-stable (Win 10 Release NVIDIA) due to a
crash in DCLayerTree::InitializeVideoProcessor(), probably the same root
cause as similar issue in the wild.  Mark the test as flaky with retry
on failure for now.

Bug: 978181
Change-Id: I3ddd97664dcb06f4fac094dcf2ed777a6384610f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1677509
Commit-Queue: Sunny Sachanandani <sunnyps@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Auto-Submit: Sunny Sachanandani <sunnyps@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672331}

content/test/gpu/gpu_tests/test_expectations/trace_test_expectations.txt

@@ -38,3 +38,4 @@ crbug.com/937545 [ win10 nvidia ] VideoPathTraceTest_DirectComposition_ComplexOv
 # Flaky for unknown reason.
 crbug.com/960007 [ win10 nvidia ] OverlayModeTraceTest_DirectComposition_Nonroot [ RetryOnFailure ]
 crbug.com/969117 [ win10 intel ] OverlayModeTraceTest_DirectComposition_Video_MP4_FourColors_Aspect_4x3 [ RetryOnFailure ]
+crbug.com/978181 [ win10 nvidia ] OverlayModeTraceTest_DirectComposition_Underlay [ RetryOnFailure ]