[WebApps]Treat the web app manifest minimal-ui display mode as installable

This CL treats the 'minimal-ui' display mode in the web app manifest as
installable if the dedicated feature flag is enabled.

Currently Minimal-UI PWAs added to home screen with the flag enabled
will behave as PWAs with Standalone display mode. A follow up patch
(crrev.com/c/566168) will introduce changes to implement Minimal-UI UX
once launched.

Bug: 604390
Change-Id: Id263cc76b6e83f201b4cd3f845f0e871c6357dca
Reviewed-on: https://chromium-review.googlesource.com/611843
Commit-Queue: Piotr Swigon <[email protected]>
Reviewed-by: Dominick Ng <[email protected]>
Cr-Commit-Position: refs/heads/master@{#494677}
4 files changed