I am unsure whether we want to delete WebApk.Install.InstallSource

It is interesting to see what the distribution is for WebAPK installs specifically between installs from the menu and installs from the banner. For instance, it is possible that adding a shortcut from the homescreen becomes much more popular for "webpage bookmarks" but has no impact at all for WebAPKs.

#2: this metric is only recorded for PWAs, and not for home screen bookmarks. It is recorded at exactly the same time as WebApk.Install.InstallSource, but is recorded on all platforms.

It turns out that https://chromium-review.googlesource.com/c/chromium/src/+/802336 (in M65) unintentionally removed the call to webapk::TrackInstallSource (it was intended that both methods be called, but somehow I missed in review that the call was deleted). I'll send a CL to remove webapk::TrackInstallSource since it isn't being called now; we'll just have to tolerate a slight discontinuity in our metrics. :(

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/10125460b69e7b3dee07fe121e572e9e8dc9317e

commit 10125460b69e7b3dee07fe121e572e9e8dc9317e
Author: Dominick Ng <dominickn@chromium.org>
Date: Tue Mar 20 00:04:56 2018

Remove the WebApk.Install.InstallSource metric.

This Android-specific metric was replaced by the cross-platform
Webapp.Install.InstallEvent in M65. It was intended that these metrics exist
alongside each other, but WebApk.Install.InstallSource was inadvertently
removed from being recorded in https://crrev.com/c/802336, meaning that data
is no longer collected as of M65.

BUG= 790788 

Change-Id: I0fc4724cc19fd4103e192f1e5caf6a9347831250
Reviewed-on: https://chromium-review.googlesource.com/967906
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Peter Kotwicz <pkotwicz@chromium.org>
Commit-Queue: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544216}
[modify] https://crrev.com/10125460b69e7b3dee07fe121e572e9e8dc9317e/chrome/browser/android/webapk/webapk_metrics.cc
[modify] https://crrev.com/10125460b69e7b3dee07fe121e572e9e8dc9317e/chrome/browser/android/webapk/webapk_metrics.h
[modify] https://crrev.com/10125460b69e7b3dee07fe121e572e9e8dc9317e/tools/metrics/histograms/enums.xml
[modify] https://crrev.com/10125460b69e7b3dee07fe121e572e9e8dc9317e/tools/metrics/histograms/histograms.xml