Suppress Flaky WebGL2 test

deqp/data/gles3/shaders/conversions.html
on Win Nvidia GL passthrough

TBR=kbr@chromium.org

Bug: 887578
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: Id07d0302853e6f6340e305508098ab756611e28b
Reviewed-on: https://chromium-review.googlesource.com/1236507
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592893}

content/test/gpu/gpu_tests/webgl2_conformance_expectations.py

@@ -233,7 +233,8 @@ class WebGL2ConformanceExpectations(WebGLConformanceExpectations):
         ['win', 'nvidia', 'opengl', 'passthrough'], bug=830046)
     self.Flaky('conformance2/transform_feedback/switching-objects.html',
         ['win', 'nvidia', 'opengl', 'no_passthrough'], bug=832238)
-
+    self.Flaky('deqp/data/gles3/shaders/conversions.html',
+        ['win', 'nvidia', 'opengl', 'passthrough'], bug=887578)
     self.Flaky('deqp/functional/gles3/transformfeedback/*',
         ['win', ('nvidia', 0x1cb3), 'opengl'], bug=822733)