The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/000cea89869e0276d886fe541b3ff050aba9527f

commit 000cea89869e0276d886fe541b3ff050aba9527f
Author: bmcquade <bmcquade@chromium.org>
Date: Thu May 25 13:00:55 2017

Factor management of metrics updates into its own class.

With the recent changes to merge paint timings across all frames in a page, we
have learned that first paint timings can arrive out of order.

This change factors IPC update management into its own class, which will
make it easier to refactor this new class to also support buffering of
cross frame paints, should we decide to implement that in a subsequent change.

BUG= 725610 

Review-Url: https://codereview.chromium.org/2904533002
Cr-Commit-Position: refs/heads/master@{#474635}

[modify] https://crrev.com/000cea89869e0276d886fe541b3ff050aba9527f/chrome/browser/BUILD.gn
[modify] https://crrev.com/000cea89869e0276d886fe541b3ff050aba9527f/chrome/browser/page_load_metrics/metrics_web_contents_observer.cc
[add] https://crrev.com/000cea89869e0276d886fe541b3ff050aba9527f/chrome/browser/page_load_metrics/page_load_metrics_update_dispatcher.cc
[add] https://crrev.com/000cea89869e0276d886fe541b3ff050aba9527f/chrome/browser/page_load_metrics/page_load_metrics_update_dispatcher.h
[modify] https://crrev.com/000cea89869e0276d886fe541b3ff050aba9527f/chrome/browser/page_load_metrics/page_load_tracker.cc
[modify] https://crrev.com/000cea89869e0276d886fe541b3ff050aba9527f/chrome/browser/page_load_metrics/page_load_tracker.h
[modify] https://crrev.com/000cea89869e0276d886fe541b3ff050aba9527f/chrome/common/BUILD.gn
[add] https://crrev.com/000cea89869e0276d886fe541b3ff050aba9527f/chrome/common/page_load_metrics/page_load_metrics_constants.h
[modify] https://crrev.com/000cea89869e0276d886fe541b3ff050aba9527f/chrome/renderer/page_load_metrics/page_timing_metrics_sender.cc