Mark http/tests/navigation/pending-location-change-assert.html flaky.

BUG=381730
TBR=abarth@chromium.org

Review URL: https://codereview.chromium.org/322553002

git-svn-id: svn://svn.chromium.org/blink/trunk@175675 bbb929c8-8fbe-4397-9dbb-9b2b20218538