[Extensions] Temporarily disable a ToolbarActionsBar test

BrowserActionsBarRedesignBrowserTest.OverflowedBrowserActionPopupTest is
flaky on Mac bots. Disabling while investigating.

Review URL: https://codereview.chromium.org/1111073003

Cr-Commit-Position: refs/heads/master@{#327391}

chrome/browser/ui/toolbar/browser_actions_bar_browsertest.cc

@@ -411,7 +411,7 @@ IN_PROC_BROWSER_TEST_F(BrowserActionsBarBrowserTest, BrowserActionPopupTest) {
 #define MAYBE_OverflowedBrowserActionPopupTest OverflowedBrowserActionPopupTest
 #endif
 IN_PROC_BROWSER_TEST_F(BrowserActionsBarRedesignBrowserTest,
-                       OverflowedBrowserActionPopupTest) {
+                       MAYBE_OverflowedBrowserActionPopupTest) {
   scoped_ptr<BrowserActionTestUtil> overflow_bar =
       browser_actions_bar()->CreateOverflowBar();