Re-add expected failure of WebGL test get-extension.

TBR=zmo@chromium.org
NOTRY=true

BUG=602688
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_optional_gpu_tests_rel;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

Review-Url: https://codereview.chromium.org/2866453002
Cr-Commit-Position: refs/heads/master@{#469346}

content/test/gpu/gpu_tests/webgl_conformance_expectations.py

@@ -113,6 +113,8 @@ class WebGLConformanceExpectations(GpuTestExpectations):
         ['passthrough'], bug=679696)
     self.Fail('conformance/extensions/ext-disjoint-timer-query.html',
         ['passthrough'], bug=1523) # angle bug ID
+    self.Fail('conformance/extensions/get-extension.html',
+        ['passthrough'], bug=682745)
     self.Fail('conformance/extensions/oes-texture-float.html',
         ['passthrough'], bug=1523) # angle bug ID
     self.Fail('conformance/extensions/oes-texture-float-linear.html',