Suppress conformance/glsl/variables/gl-frontfacing.html flake.

Failing intermittently on Windows 10 / Quadro P400 / D3D9 /
passthrough command decoder. Mark it flaky for both passthrough and
non-passthrough.

TBR=zmo@chromium.org, geofflang@chromium.org

Bug: 825416
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;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: Ica54a091b5d447be3a3da71fd022a64f3b188b22
Reviewed-on: https://chromium-review.googlesource.com/979152
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546718}

content/test/gpu/gpu_tests/webgl_conformance_expectations.py

@@ -220,6 +220,8 @@ class WebGLConformanceExpectations(GpuTestExpectations):
         ['win10', ('nvidia', 0x1cb3), 'd3d9'], bug=680754)
     self.Flaky('conformance/extensions/oes-texture-float-with-video.html',
         ['win10', ('nvidia', 0x1cb3), 'd3d9'], bug=750896)
+    self.Flaky('conformance/glsl/variables/gl-frontfacing.html',
+        ['win10', ('nvidia', 0x1cb3), 'd3d9'], bug=825416)
     self.Fail('conformance/ogles/GL/atan/atan_001_to_008.html',
         ['win10', ('nvidia', 0x1cb3), 'd3d9'], bug=737018)
     self.Fail('conformance/ogles/GL/cos/cos_001_to_006.html',