Disable AutomationApiTest.Mixin on Windows due to flakiness

TBR=aboxhall@chromium.org
BUG=467921

Review URL: https://codereview.chromium.org/1063163003

Cr-Commit-Position: refs/heads/master@{#325274}

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

@@ -200,8 +200,8 @@ IN_PROC_BROWSER_TEST_F(AutomationApiTest, Find) {
       << message_;
 }
 
-// Flaky on Linux only. http://crbug.com/467921
-#if defined(OS_LINUX)
+// Flaky on Linux and Windows. http://crbug.com/467921
+#if defined(OS_LINUX) || defined(OS_WIN)
 #define MAYBE_Mixins DISABLED_Mixins
 #else
 #define MAYBE_Mixins Mixins