Suppress Mac WebGL conformance uniforms uniform-samplers-test

This test is disabled due to a regression on Mac FYI Retina Debug (NVIDIA)
bot since build 2964

NOTRY=true

Bug: 871352
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: Ida3ec7f78ee39c2578f0ee802101559de11c92d9
Reviewed-on: https://chromium-review.googlesource.com/1163993
Commit-Queue: Maggie Chen <magchen@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580964}

content/test/gpu/gpu_tests/webgl_conformance_expectations.py

@@ -441,6 +441,8 @@ class WebGLConformanceExpectations(GpuTestExpectations):
         ['mac', ('nvidia', 0xfe9)], bug=635081)
     self.Fail('conformance/glsl/bugs/init-array-with-loop.html',
         ['mac', ('nvidia', 0xfe9)], bug=784817)
+    self.Fail('conformance/uniforms/uniform-samplers-test.html',
+        ['mac', 'debug', ('nvidia', 0xfe9)], bug=871352)
 
     # Linux failures
     self.Fail('conformance/extensions/webgl-compressed-texture-astc.html',