Add kOpenerNavigationDownloadCrossOriginNoGesture to UKM whitelist

Bug: 632514
Change-Id: Ibd09c4d8635873e02f9b484ec720b71ae6e3588f
Reviewed-on: https://chromium-review.googlesource.com/c/1399521
Reviewed-by: Bryan McQuade <bmcquade@chromium.org>
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#620513}