Upgrade XFB failing expectation.

This flaky expectation for "too-small-buffers" was not sufficient
to prevent try jobs from failing. Upgrade it to "Fail" so it will
prevent flaky retries.

Examples:
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Win10%20FYI%20Release%20%28NVIDIA%29/182
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Win10%20FYI%20Release%20%28NVIDIA%29/181

Bug: 832238
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: I646cb3815d93c11f3cb39d1e919f7db2283e4ba5
Tbr: kbr@chromium.org
No-Try: True
Reviewed-on: https://chromium-review.googlesource.com/1014734Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551312}

content/test/gpu/gpu_tests/webgl2_conformance_expectations.py

@@ -426,7 +426,7 @@ class WebGL2ConformanceExpectations(WebGLConformanceExpectations):
     # These Transform Feedback tests seem flaky on ANGLE/GL with passthrough.
     self.Flaky('conformance2/transform_feedback/switching-objects.html',
         ['passthrough', 'opengl'], bug=832238)
-    self.Flaky('conformance2/transform_feedback/too-small-buffers.html',
+    self.Fail('conformance2/transform_feedback/too-small-buffers.html',
         ['passthrough', 'opengl'], bug=832238)
 
     # Passthrough command decoder / OpenGL / Intel