The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/06c594634073ae8cc37099fadd15cd02ae035398

commit 06c594634073ae8cc37099fadd15cd02ae035398
Author: bmcquade <bmcquade@chromium.org>
Date: Mon Apr 10 14:46:19 2017

Various page_load_metrics cleanups and improvements

* break dependency on RenderViewHost where possible
* log more detailed error codes when an invalid timing IPC is received
* log whether each page load started in the foreground
* count the number of prerenders

The latter three are important in improving our understanding of metrics
differences we are seeing with browser side navigation enabled.

BUG= 709140 

Review-Url: https://codereview.chromium.org/2804093002
Cr-Commit-Position: refs/heads/master@{#463263}

[modify] https://crrev.com/06c594634073ae8cc37099fadd15cd02ae035398/chrome/browser/page_load_metrics/metrics_web_contents_observer.cc
[modify] https://crrev.com/06c594634073ae8cc37099fadd15cd02ae035398/chrome/browser/page_load_metrics/metrics_web_contents_observer_unittest.cc
[modify] https://crrev.com/06c594634073ae8cc37099fadd15cd02ae035398/chrome/browser/page_load_metrics/page_load_metrics_browsertest.cc
[modify] https://crrev.com/06c594634073ae8cc37099fadd15cd02ae035398/chrome/browser/page_load_metrics/page_load_tracker.cc
[modify] https://crrev.com/06c594634073ae8cc37099fadd15cd02ae035398/chrome/browser/page_load_metrics/page_load_tracker.h
[modify] https://crrev.com/06c594634073ae8cc37099fadd15cd02ae035398/tools/metrics/histograms/histograms.xml