Mark flaky WebGL2 test on Mac NVidia

WebglConformance_conformance2_textures_canvas_tex_3d_r11f_g11f_b10f_rgb_unsigned_int_10f_11f_11f_rev
is flaky.

Bug: 934556
Change-Id: I19e9f01ec63a9ac2e629fde024997f56682b4e31
Reviewed-on: https://chromium-review.googlesource.com/c/1481120
Auto-Submit: Eric Karl <ericrk@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634501}

content/test/gpu/gpu_tests/webgl2_conformance_expectations.py

@@ -677,6 +677,10 @@ class WebGL2ConformanceExpectations(WebGLConformanceExpectations):
     self.Fail('conformance2/glsl3/tricky-loop-conditions.html',
         ['mac', ('nvidia', 0xfe9)], bug=929398)
 
+    self.Flaky('conformance2/textures/canvas_sub_rectangle/' +
+        'tex-3d-r11f_g11f_b10f-rgb-unsigned_int_10f_11f_11f_rev.html',
+        ['mac', ('nvidia', 0xfe9)], bug=934556)
+
     # When these fail on this configuration, they fail multiple times in a row.
     self.Fail('deqp/functional/gles3/shaderoperator/*',
         ['mac', 'nvidia'], bug=756537)