Retry flaky webgl2_conformance_tests on Mac Nvidia.

Three tests are timing out occasionally on Mac FYI Retina Release
(NVIDIA). Mark as RetryOnFailure to suppress failures as Mac Nvidia
drivers are unlikely to get fixed.

Bug: 1022831
Change-Id: If241719809f135546d89ca13fb69e4107a06cb9c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1906879Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Commit-Queue: kylechar <kylechar@chromium.org>
Auto-Submit: kylechar <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713953}

content/test/gpu/gpu_tests/test_expectations/webgl2_conformance_expectations.txt

@@ -396,6 +396,9 @@ crbug.com/952282 [ mac nvidia-0xfe9 ] conformance2/rendering/framebuffer-unsuppo
 crbug.com/965209 [ mac nvidia-0xfe9 ] conformance2/samplers/multi-context-sampler-test.html [ RetryOnFailure ]
 crbug.com/756537 [ mac nvidia ] deqp/functional/gles3/shaderoperator/* [ Failure ]
 crbug.com/907599 [ mac nvidia debug ] conformance/textures/video/tex-2d-rgb-rgb-unsigned_short_5_6_5.html [ RetryOnFailure ]
+crbug.com/1022831 [ mac nvidia-0xfe9 ] deqp/functional/gles3/vertexarrays/single_attribute.usage.stream_draw.html [ RetryOnFailure ]
+crbug.com/1022831 [ mac nvidia-0xfe9 ] deqp/functional/gles3/vertexarrays/single_attribute.usage.dynamic_copy.html [ RetryOnFailure ]
+crbug.com/1022831 [ mac nvidia-0xfe9 ] deqp/functional/gles3/shaderloop_while.html [ RetryOnFailure ]
 
 # Mac AMD
 # TODO(kbr): uncomment the following two exepectations after test