Make org.chromium.chrome.browser.FullscreenActivityTest#testNoIntentWhenInBackground flaky

Chromium Build Sheriffing.

TBR=yusufo@chromium.org

Bug: 906831
Change-Id: Iaa9db6b2cd31ece2c0a84159418ced80d65557cb
Reviewed-on: https://chromium-review.googlesource.com/c/1344610


Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609908}