Disable WebviewInExtensionTest.testWebviewInExtension on macOS.

telemetry/telemetry/internal/browser/extension_unittest.py

Index: telemetry/telemetry/internal/browser/extension_unittest.py
diff --git a/telemetry/telemetry/internal/browser/extension_unittest.py b/telemetry/telemetry/internal/browser/extension_unittest.py
index 0c85f173c1a2cd8b1eeaa40135e976b9bb92f64a..ae599f72492f390e52e73e7c8fefef85e3f07ef2 100644
--- a/telemetry/telemetry/internal/browser/extension_unittest.py
+++ b/telemetry/telemetry/internal/browser/extension_unittest.py
@@ -203,7 +203,8 @@ class ComponentExtensionTest(unittest.TestCase):
 class WebviewInExtensionTest(ExtensionTest):
 
   # Flaky on windows, hits an exception: http://crbug.com/508325
-  @decorators.Disabled('win', 'linux')
+  # Flaky on macOS too: http://crbug.com/661434
+  @decorators.Disabled('win', 'linux', 'mac')
   def testWebviewInExtension(self):
     """Tests GetWebviewContext() for a web app containing <webview> element."""
     if not self.CreateBrowserWithExtension('webview_app'):