I suggest preventing users from creating WebAPKs for http://localhost. This is not a very important scenario so I am tempted to go with the easiest fix.

> This is not a very important scenario

Perhaps it might be important for development/testing?

After offline discussion, it was decided that it is good to enable creating WebAPKs for http://localhost URLs. The new intent of this bug is to make it possible launch WebAPKs created for http://localhost URLs in WebAPK mode.

Repro steps:
1) Install a WebAPK via add-to-homescreen from the app menu for a http://localhost page. I checked out https://github.com/gauntface/simple-push-demo and navigated to http://localhost/push_demo
2) Launch to installed WebAPK
Expected:
WebAPK is launched fullscreen. A splash screen should be shown when the app is opened
Actual:
The URL is launched in a new browser tab

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/91697d2c952534944300a1dee86a90d135c736a1

commit 91697d2c952534944300a1dee86a90d135c736a1
Author: pkotwicz <pkotwicz@chromium.org>
Date: Fri Feb 10 16:51:47 2017

Enable opening http://localhost WebAPK in WebAPK mode

Chrome launches WebAPKs in WebAPK mode only if they are valid WebAPKs. One of
the validity requirements is that the WebAPK's start URL be within the scope of
the WebAPK's intent filter. This CL makes the intent filter's scheme
customizable by the WebAPK server.

BUG= 655438 

Review-Url: https://codereview.chromium.org/2684373002
Cr-Commit-Position: refs/heads/master@{#449635}

[modify] https://crrev.com/91697d2c952534944300a1dee86a90d135c736a1/chrome/android/webapk/shell_apk/AndroidManifest.xml
[modify] https://crrev.com/91697d2c952534944300a1dee86a90d135c736a1/chrome/android/webapk/shell_apk/BUILD.gn