Blacklist KeyboardOverlayUIBrowserTest on mash

Blacklist this flaky test for single_process_mash_browser_test.
Keyboard overlay is slated to be removed soon.

Bug: 884216
Test: verify filter file locally
Change-Id: I48c7cc0de3648c9a5445d502f3fe08f436631605
Reviewed-on: https://chromium-review.googlesource.com/1227139
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Commit-Queue: Vladislav Kaznacheev <kaznacheev@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591402}