Expand WebGL 2 XFB test expectation again.

It seems 'opengl' does not cover Mac configurations. Rephrase the
expectation.

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: I493a86bc9c1c594f64fae6de6b11adbde4b33007
Tbr: kbr@chromium.org
No-Try: True
Reviewed-on: https://chromium-review.googlesource.com/1022731Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552478}

content/test/gpu/gpu_tests/webgl2_conformance_expectations.py

@@ -49,6 +49,8 @@ class WebGL2ConformanceExpectations(WebGLConformanceExpectations):
         bug=757097)
 
     # Flakes heavily on many OpenGL configurations
+    self.Fail('conformance2/transform_feedback/too-small-buffers.html',
+        ['no_angle'], bug=832238)
     self.Fail('conformance2/transform_feedback/too-small-buffers.html',
         ['opengl'], bug=832238)