Suppress conformance/extensions/webgl-draw-buffers

The failure is in new Linux Intel webgl conformance (opengl)

I think it's been exposed with the fix in
https://codereview.chromium.org/1883153003

BUG=586536
TEST=new linux intel bots
NOTRY=true
NOTREECHECKS=true
TBR=bajones@chromium.org,kbr@chromium.org
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_optional_gpu_tests_rel;tryserver.chromium.mac:mac_optional_gpu_tests_rel;tryserver.chromium.win:win_optional_gpu_tests_rel

Review URL: https://codereview.chromium.org/1896693002

Cr-Commit-Position: refs/heads/master@{#387814}

content/test/gpu/gpu_tests/webgl_conformance_expectations.py

@@ -222,6 +222,8 @@ def SetExpectations(self):
         ['linux', 'intel', 'opengl'], bug=598924)
     self.Fail('conformance/uniforms/gl-uniform-arrays.html',
         ['linux', 'debug', ('intel', 0x412)], bug=604140)
+    self.Fail('conformance/extensions/webgl-draw-buffers.html',
+        ['linux', ('intel', 0x412), 'opengl'], bug=586536)
 
     # Android failures
     self.Fail('deqp/data/gles2/shaders/constants.html',