GitLab

For a little while we have been recording startup metrics based on main
entry point time using two techniques:
* (old way) by converting process creation wall time using
  StartupTimeToTimeTicks()
* (new way) by calling base::TimeTicks::Now()

The new way is theoretically cleaner and removes a few possible skews
(see details in the bug). During a few months of overlap we have been
computing these two histograms for the identical time interval:
* (old way) Startup.BrowserMessageLoopStartTimeFromMainEntry2
* (new way) Startup.BrowserMessageLoopStartTimeFromMainEntry3

On Windows the skew between the two is super tiny (2-3 UMA samples with
difference during the day).

On Android the difference is between 0.5% and 0.7% for the same day for
various quantiles. The overflow bucket (1 hour and more) for the new
histogram is 3.5x smaller - nice. Also the new histogram looks a tiny
bit smoother on Android. Overall the shape of the distribution does not
change our subjective perception of the startup metrics on Android.

Based on these observations this change proposes to:
* Switch the remaining calculations based on the main entry point to the
  new way - calling base::TimeTicks::Now() (no conversion to ticks, not
  using the wallclock)
* Avoid renaming the histograms because the expected difference is
  likely not worth worrying
* Celebrate cleanup :)

Bug: 797762
Change-Id: I5c32541280c359d6571cbcc5c6e7500b79002011
Reviewed-on: https://chromium-review.googlesource.com/1104696Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: Egor Pasko <pasko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568189}