Implement WebApp LaunchCauseMetrics

For WebApps we primarily care about distinguishing WebApps installed via
Chrome, vs installed via other sources, as this hints to whether the
user knew they were using Chrome.

I also implement HOME_SCREEN_SHORTCUT metrics here - they're part of
a2hs functionality.

Bug: 1163961
Change-Id: Ib898645aa97374fc3fcd80b29acac385cae985fb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2692515
Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Commit-Queue: Michael Thiessen <mthiesse@chromium.org>
Cr-Commit-Position: refs/heads/master@{#854343}