Disable Metal test shader in WebGL conformance tests.

This is occasionally causing GPU process crashes, leading to random
flakiness throughout the test suite. This will be handled more
generally soon.

Bug: 1085899, 1088465
Tbr: bsheedy@chromium.org
Change-Id: I90b40c7232a1dc0238e8a7792b577a4290b84535
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2225979Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Auto-Submit: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#773981}

content/test/gpu/gpu_tests/webgl_conformance_integration_test.py

@@ -341,6 +341,10 @@ class WebGLConformanceIntegrationTest(gpu_integration_test.GpuIntegrationTest):
         # TODO(crbug.com/830901): see whether disabling this feature
         # makes the WebGL video upload tests reliable again.
         '--disable-features=UseSurfaceLayerForVideo',
+        # TODO(crbug.com/1085899): promote this to an argument that's
+        # specified for all of the GPU tests, not just the WebGL
+        # conformance tests.
+        '--disable-metal-test-shaders',
     ]
     # Note that the overriding of the default --js-flags probably
     # won't interact well with RestartBrowserIfNecessaryWithArgs, but