Disable AutomationApiTest.Mixin on Linux due to flakiness

TBR=aboxhall@chromium.org
BUG=467921

Review URL: https://codereview.chromium.org/1014543006

Cr-Commit-Position: refs/heads/master@{#320919}

chrome/browser/extensions/api/automation/automation_apitest.cc

@@ -187,6 +187,12 @@ IN_PROC_BROWSER_TEST_F(AutomationApiTest, Find) {
       << message_;
 }
 
+// Flaky on Linux only. http://crbug.com/467921
+#if defined(OS_LINUX)
+#define MAYBE_Mixins DISABLED_Mixins
+#else
+#define MAYBE_Mixins Mixins
+#endif
 IN_PROC_BROWSER_TEST_F(AutomationApiTest, Mixins) {
   StartEmbeddedTestServer();
   ASSERT_TRUE(RunExtensionSubtest("automation/tests/tabs", "mixins.html"))