Suppress flaky WebGL test

WebglConformance_conformance_uniforms_uniform_default_values is flaking
on Win10 FYI Release (NVIDIA).

TBR=kbr@chromium.org

Bug: 907544
Change-Id: Ib41ff367449b0fd3c424d95fdf39f4fdcf7e4633
Reviewed-on: https://chromium-review.googlesource.com/c/1346673Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Commit-Queue: Saman Sami <samans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610119}

content/test/gpu/gpu_tests/webgl2_conformance_expectations.py

@@ -170,6 +170,8 @@ class WebGL2ConformanceExpectations(WebGLConformanceExpectations):
         ['win', 'nvidia', 'd3d11'], bug=679639)
     self.Flaky('deqp/functional/gles3/textureshadow/*.html',
         ['win', 'nvidia', 'd3d11'], bug=735464)
+    self.Flaky('conformance/uniforms/uniform-default-values.html',
+        ['win', 'nvidia', 'vulkan', 'passthrough'], bug=907544)
 
     # Win / NVIDIA Quadro P400 / D3D11 flaky failures
     self.Fail('deqp/data/gles3/shaders/functions.html',