Mark test_resource_timing_cross_origin_redirect_chain_allow_timing as flaky.

BUG=431643
TBR=senorblanco@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/710013002

git-svn-id: svn://svn.chromium.org/blink/trunk@185021 bbb929c8-8fbe-4397-9dbb-9b2b20218538