This change updates the flow for showing the app banner when the
experimental app banner is enabled. Specifically, if the experiment flow
is enabled:
- by default app banners will not be shown, instead the web app needs to
call event.prompt
- preventDefault does not need to be called for e.prompt to work
- a gesture is required to call e.prompt

Bug: 729919
Change-Id: I6e5443b663c6139d8fbb8c9121839143470cdd37
Reviewed-on: https://chromium-review.googlesource.com/569842


Reviewed-by: Dominick Ng <dominickn@chromium.org>
Commit-Queue: Ben Wells <benwells@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487708}