UKM collection doc: https://docs.google.com/document/d/1cu4NhvN5_aexi40DdLUV7l_3q4W31d-t9ULsHzxU0Gk/edit#

We realized we can tell whether the page was reloaded already, but not whether the load was user-initiated. This would be great to have for a lot of use cases, including reloads. Changing this issue to be about adding a boolean to PageLoad UKM to indicate whether the page load was user-initiated.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/496d10a4755317cfeac6349bf88d6270416c37e3

commit 496d10a4755317cfeac6349bf88d6270416c37e3
Author: Annie Sullivan <sullivan@chromium.org>
Date: Tue Jan 22 23:23:51 2019

Add a UserInitiated boolean to PageLoad UKM

This counts a load as user-initiated if it's started in the browser, or
it's started in the renderer and we can tie it back to the original
input event. There are some notable missing cases in the renderer, like
back/forward and reload.

Bug: 921710
Change-Id: Ibc034fbfa077a73148c9cc93032e8dc4a57dae5f
Reviewed-on: https://chromium-review.googlesource.com/c/1423110
Commit-Queue: Annie Sullivan <sullivan@chromium.org>
Reviewed-by: Bryan McQuade <bmcquade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624982}
[modify] https://crrev.com/496d10a4755317cfeac6349bf88d6270416c37e3/chrome/browser/page_load_metrics/observers/ukm_page_load_metrics_observer.cc
[modify] https://crrev.com/496d10a4755317cfeac6349bf88d6270416c37e3/chrome/browser/page_load_metrics/observers/ukm_page_load_metrics_observer_unittest.cc
[modify] https://crrev.com/496d10a4755317cfeac6349bf88d6270416c37e3/tools/metrics/ukm/ukm.xml