I think we need two metrics:
1. WebApk.Install.ShellApkVersion &
2. WebApk.Update.ShellApkVersion

The "Install" one records the shell APK version of a new installed WebAPK.
The "Update" one records the shell APK version of a WebAPK that was updated from an old version.

Therefore, we can see how many existing WebAPK has updated to version 3:
"Number in UPDATE metrics with version = 3" % ("Number in INSTALL metrics with version = 1" + "Number in INSTALL metrics with version = 2").

Further more, we can see how many existing WebAPKs has updated from version 2 to version 3:
"Number in UPDATE metrics with version = 3" % ("Number in UPDATE metrics with version = 2" + "Number in INSTALL metrics with version = 2").

However you could only do that analysis for the latest version since some users may skip versions and the update doesn't tell you which version they came from.

Agree, the first calculation seems to be a better way to use the data.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7ff54f9912ee3dd2132e0fc7c1e9bdadc06b46b1

commit 7ff54f9912ee3dd2132e0fc7c1e9bdadc06b46b1
Author: pkotwicz <pkotwicz@chromium.org>
Date: Wed May 10 23:57:05 2017

[Android WebAPKs] Add UMA metric for ShellAPK version

This CL adds a UMA metric for the ShellAPK version. The metric will tell us the
percentage of users with WebAPKs with a given ShellAPK version.

There is a separate metric for "Google Play WebAPKs" and "Browser WebAPKs". We
expect the ShellAPK for "Unbound WebAPKs" to be permanently out of date. We
hope that the ShellAPK for "Browser WebAPKs" stays up to date

BUG= 691743 

Review-Url: https://codereview.chromium.org/2862493002
Cr-Commit-Position: refs/heads/master@{#470756}

[modify] https://crrev.com/7ff54f9912ee3dd2132e0fc7c1e9bdadc06b46b1/chrome/android/java/src/org/chromium/chrome/browser/metrics/WebApkUma.java
[modify] https://crrev.com/7ff54f9912ee3dd2132e0fc7c1e9bdadc06b46b1/chrome/android/java/src/org/chromium/chrome/browser/webapps/WebApkActivity.java
[modify] https://crrev.com/7ff54f9912ee3dd2132e0fc7c1e9bdadc06b46b1/tools/metrics/histograms/histograms.xml

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7586aa67fae852113735474c358205cef090e843

commit 7586aa67fae852113735474c358205cef090e843
Author: yfriedman <yfriedman@chromium.org>
Date: Fri May 19 15:52:11 2017

Fix metric collection for shell apk version.

The implicit separator is '_' but the metric is recorded with '.'
BUG= 691743 

Review-Url: https://codereview.chromium.org/2886343003
Cr-Commit-Position: refs/heads/master@{#473203}

[modify] https://crrev.com/7586aa67fae852113735474c358205cef090e843/tools/metrics/histograms/histograms.xml