Suppress SiteDetailsBrowserTest.IsolateExtensions in mash_browser_tests

TBR=jamescook,nick

Bug: 671891
Change-Id: Ic0c49bfb7f012fa6f8bd3e5150f57274d439d073
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/1159927
Commit-Queue: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580060}

testing/buildbot/filters/mash.browser_tests.filter

@@ -310,4 +310,9 @@
 # https://crbug.com/814675
 -ChromeOSInfoPrivateTest.StylusSeen
 
+# Has become super flaky on linux-chromeos-rel bot. Note that the test itself
+# is also flaky on Windows and Mac even without "mash":
+# https://crbug.com/671891
+-SiteDetailsBrowserTest.IsolateExtensions
+
 # See comment at top of file regarding adding test exclusions.