Temporarily suppress some ContextLost.WebGLContextLostInHiddenTab failures.

Suppress failures on Retina Macs. These failures are kicking valid
jobs out of the commit queue. Tested suppression locally.

BUG=422803
TBR=zmo@chromium.org,bajones@chromium.org

Review URL: https://codereview.chromium.org/650773002

Cr-Commit-Position: refs/heads/master@{#299264}

content/test/gpu/gpu_tests/context_lost_expectations.py

@@ -26,3 +26,6 @@ class ContextLostExpectations(test_expectations.TestExpectations):
 
     self.Fail('GpuCrash.GPUProcessCrashesExactlyOnce',
               ['win'], bug=392891)
+
+    self.Fail('ContextLost.WebGLContextLostInHiddenTab',
+              ['mac', ('nvidia', 0xfe9)], bug=422803)