Return the userChoice promise from beforeinstallpromptevent.prompt().

This corrects a spec violation in the implementation of
beforeinstallpromptevent, where the prompt() method was not returning
the event's userChoice promise. Web tests are updated to test this.

BUG=965939

Change-Id: I6e1bd2f83a287d4e7888a873984760bdecab0161
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1626444
Commit-Queue: Mounir Lamouri <mlamouri@chromium.org>
Auto-Submit: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662659}
2 files changed