The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7894b9acb1c4de15bf778042b826a72f3e358069

commit 7894b9acb1c4de15bf778042b826a72f3e358069
Author: bmcquade <bmcquade@chromium.org>
Date: Thu Mar 02 21:31:10 2017

Add UKM initial_url and session_id fields.

This change adds support for logging the following new fields in UKM:
1. initial_url: the initial URL of a source, in cases where the URL of a source
   changes
2. session_id: a counter that's incremented for each browsing session (similar
   to the UMA session_id field)

Logging of both of these fields is gated on field trial params, and is disabled
by default. We'll start logging once the server-side changes for these fields are
in place.

BUG= 697944 

Review-Url: https://codereview.chromium.org/2722983006
Cr-Commit-Position: refs/heads/master@{#454382}

[modify] https://crrev.com/7894b9acb1c4de15bf778042b826a72f3e358069/components/metrics/proto/ukm/report.proto
[modify] https://crrev.com/7894b9acb1c4de15bf778042b826a72f3e358069/components/metrics/proto/ukm/source.proto
[modify] https://crrev.com/7894b9acb1c4de15bf778042b826a72f3e358069/components/ukm/BUILD.gn
[modify] https://crrev.com/7894b9acb1c4de15bf778042b826a72f3e358069/components/ukm/test_ukm_service.cc
[modify] https://crrev.com/7894b9acb1c4de15bf778042b826a72f3e358069/components/ukm/ukm_pref_names.cc
[modify] https://crrev.com/7894b9acb1c4de15bf778042b826a72f3e358069/components/ukm/ukm_pref_names.h
[modify] https://crrev.com/7894b9acb1c4de15bf778042b826a72f3e358069/components/ukm/ukm_service.cc
[modify] https://crrev.com/7894b9acb1c4de15bf778042b826a72f3e358069/components/ukm/ukm_service.h
[modify] https://crrev.com/7894b9acb1c4de15bf778042b826a72f3e358069/components/ukm/ukm_service_unittest.cc
[modify] https://crrev.com/7894b9acb1c4de15bf778042b826a72f3e358069/components/ukm/ukm_source.cc
[modify] https://crrev.com/7894b9acb1c4de15bf778042b826a72f3e358069/components/ukm/ukm_source.h