Supress WebGL2 failures.

Supress new failures from this roll: https://codereview.chromium.org/2331823002

TBR=zmo@chromium.org
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel;master.tryserver.chromium.android:android_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2337493002
Cr-Commit-Position: refs/heads/master@{#417952}

content/test/gpu/gpu_tests/webgl2_conformance_expectations.py

@@ -334,6 +334,8 @@ class WebGL2ConformanceExpectations(WebGLConformanceExpectations):
         ['mac', 'amd'], bug=643866)
     self.Fail('conformance2/glsl3/bool-type-cast-bug-uint-ivec-uvec.html',
         ['mac', 'amd'], bug=643866)
+    self.Fail('conformance2/textures/misc/tex-mipmap-levels.html',
+        ['mac', 'amd'], bug=483282)
     self.Fail('deqp/functional/gles3/shadercommonfunction.html',
         ['mac', 'amd'], bug=643866)