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This CL adds three new fields to UkmSource. A follow-up will propagate
changes to the underlying proto fields.

Bug: 873316
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Ic691221c8165df7cee16b089407d8ddd346e78c3
Reviewed-on: https://chromium-review.googlesource.com/1175033
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Robert Kaplow (slow) <rkaplow@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585824}