GitLab

This CL:
1. Adds method to ContentBrowserClient which allows logging blink
   UseCounters scoped to a given RenderFrameHost.

2. Uses this new API to log UseCounters for opener navigations resulting
   in downloads.

Bug: 632514
Change-Id: I4459281c5a104fac776a13f9f003b1d75349354f
Reviewed-on: https://chromium-review.googlesource.com/c/1324244
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Bryan McQuade <bmcquade@chromium.org>
Reviewed-by: Luna Lu <loonybear@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613351}