gpu: Mark webgl2 uniform block buffer size test as flaky on Mac Nvidia

This test fails most of the time, but let's mark it RetryOnFailure first
and check if that's enough to make the bot go green.

TBR=zmo@chromium.org

Bug: 1027534
Change-Id: I1402fbef8539210a1930e04f4327da35a2ea40d1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1956564Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>
Commit-Queue: Sunny Sachanandani <sunnyps@chromium.org>
Cr-Commit-Position: refs/heads/master@{#722739}

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

@@ -426,6 +426,7 @@ crbug.com/907599 [ mac nvidia debug ] conformance/textures/video/tex-2d-rgb-rgb-
 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 ]
+crbug.com/1027534 [ mac nvidia-0xfe9 ] conformance2/rendering/uniform-block-buffer-size.html [ RetryOnFailure ]
 
 # Mac AMD
 # TODO(kbr): uncomment the following two exepectations after test