Disabled PopupBlockerBrowserTest.Regress427477 on Mac

This test seems flaky when disabling browser-side-navigation according to the
flakiness dashboard.

TEST=interactive_ui_tests --gtest_filter=PopupBlockerBrowserTest.Regress427477 --disable-browser-side-navigation
TBR=avi@chromium.org

Bug: 780371
Change-Id: I2cd1027ebe8ce2d46919e8a492cda078417be935
Reviewed-on: https://chromium-review.googlesource.com/748503
Commit-Queue: Makoto Shimazu <shimazu@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513081}

chrome/browser/ui/blocked_content/popup_blocker_browsertest.cc

@@ -51,6 +51,7 @@
 #include "content/public/browser/render_widget_host.h"
 #include "content/public/browser/web_contents.h"
 #include "content/public/browser/web_contents_observer.h"
+#include "content/public/common/browser_side_navigation_policy.h"
 #include "content/public/common/url_constants.h"
 #include "content/public/test/browser_test_utils.h"
 #include "content/public/test/test_navigation_observer.h"
@@ -608,6 +609,9 @@ IN_PROC_BROWSER_TEST_F(PopupBlockerBrowserTest, DenialOfService) {
 
 // Verify that an onunload popup does not show up for about:blank.
 IN_PROC_BROWSER_TEST_F(PopupBlockerBrowserTest, Regress427477) {
+  // Flaky in this case: crbug.com/780371
+  if (!content::IsBrowserSideNavigationEnabled())
+    return;
   ui_test_utils::NavigateToURL(browser(), GURL(chrome::kChromeUINewTabURL));
   ui_test_utils::NavigateToURL(browser(), GURL(url::kAboutBlankURL));