WebGL: Mark a test flaky on Linux/ANGLE.

BUG=618447
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/2345633003
Cr-Commit-Position: refs/heads/master@{#418899}

content/test/gpu/gpu_tests/webgl2_conformance_expectations.py

@@ -470,6 +470,9 @@ class WebGL2ConformanceExpectations(WebGLConformanceExpectations):
         ['linux', 'opengl'], bug=483282)
     self.Fail('conformance2/reading/read-pixels-pack-parameters.html',
         ['linux', 'opengl'], bug=483282)
+    self.Flaky('deqp/functional/gles3/texturespecification/' +
+        'random_teximage2d_2d.html',
+        ['linux', 'opengl', 'nvidia'], bug=618447)
 
     # Linux Intel
     self.Fail('deqp/functional/gles3/shaderderivate_dfdx.html',