Yaron, can you please take this on?

There shouldn't be any magic here. It's the exact same as install except we don't create a shortcut.

Xi has a CL for this

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/47b2313b91606860aa38e09910cdecb7400d1df3

commit 47b2313b91606860aa38e09910cdecb7400d1df3
Author: hanxi <hanxi@chromium.org>
Date: Wed Aug 31 22:07:24 2016

Update WebAPK using downloaded apk.

After request to update a WebAPK, a APK is downloaded by WebAPKInstaller. This CL
enables to install the downloaded WebAPK to finish the updating.

This CL depends on CL (https://codereview.chromium.org/2259553002/).

BUG= 631025 ,  637097 

Review-Url: https://codereview.chromium.org/2264753002
Cr-Commit-Position: refs/heads/master@{#415772}

[modify] https://crrev.com/47b2313b91606860aa38e09910cdecb7400d1df3/chrome/android/java/src/org/chromium/chrome/browser/webapps/WebApkInstaller.java