Suppress ContextLost.WebGLContextLostFromSelectElement flakiness on Mac 10.8.

BUG=497411

Review URL: https://codereview.chromium.org/1153173008

Cr-Commit-Position: refs/heads/master@{#333182}

content/test/gpu/gpu_tests/context_lost_expectations.py

@@ -15,3 +15,7 @@ class ContextLostExpectations(GpuTestExpectations):
     # AMD Radeon 6450
     self.Fail('ContextLost.WebGLContextLostFromGPUProcessExit',
         ['linux', ('amd', 0x6779)], bug=479975)
+
+    # Mac 10.8 (ideally should restrict this to Debug, too)
+    self.Fail('ContextLost.WebGLContextLostFromSelectElement',
+              ['mountainlion'], bug=497411)