Mark tex-mipmap-levels flaky on Mac AMD 10.13.6.

Test has been observed flaky on the commit queue.

Bug: 870856
Change-Id: I3840495ea0350417b0aa445c4bb969e05f3edc53
Tbr: enne@chromium.org
No-Try: True
Reviewed-on: https://chromium-review.googlesource.com/c/1455619
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629429}

content/test/gpu/gpu_tests/webgl2_conformance_expectations.py

@@ -445,6 +445,9 @@ class WebGL2ConformanceExpectations(WebGLConformanceExpectations):
         ['sierra', 'amd'], bug=870856)
     self.Fail('conformance2/textures/misc/tex-mipmap-levels.html',
         ['sierra', 'amd'], bug=870856)
+    # Flaky on 10.13 as well.
+    self.Flaky('conformance2/textures/misc/tex-mipmap-levels.html',
+        ['highsierra', 'amd'], bug=870856)
 
     # Regressions in 10.13
     self.Fail('deqp/functional/gles3/fbocolorbuffer/tex2d_00.html',