Upstream: Create WebAPK when user selects "Add to Home screen"

This CL makes "Add to Home Screen" create a WebAPK when a user runs Chrome with
the --enable-webapk command line flag and:
- Selects "Add to Home screen" from the app menu for a web app compatible app
OR
- Selects "Add to Home screen" from "Add to Home screen" infobar

BUG=609122

Review-Url: https://codereview.chromium.org/2031213004
Cr-Commit-Position: refs/heads/master@{#398706}
4 files changed