desktop-pwas: fix PWA file-handler icon paths

This change fixes an issue where PWA custom icons were not showing up
in the Open With menu on Windows when a PWA is registered as a file
handler on Windows.

A PWA's custom icon is located at:
<user data>/<profile>/Web Applications/<app ID>/<app name>.ico

Currently, the value of DefaultIcon as registered with Windows is:
<user data>/<profile>/Web Applications/<app ID>/<app name>.exe/<app name>.ico

Windows fails to find the icon file and instead displays a generic
icon. This is caused by GetIconFilePath() being passed
|app_launcher_path| (which has value <app name>.exe) as its
|web_app_path| parameter. This change replaces that argument with the
path to <user data>/<profile>/Web Applications/<app ID>.

Bug: 1143966
Change-Id: I4744b9866c85f570c3e9bd794d83a6abb6cd97eb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2508648
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Commit-Queue: Jesse McKenna <jessemckenna@google.com>
Cr-Commit-Position: refs/heads/master@{#823194}