Chrome Version: 60.0.3112.20 Dev
OS: Android

What steps will reproduce the problem?
(1) Turn on flag #bypass-app-banner-engagement-checks.
(2) Go to https://killer-marmot.appspot.com/web_redispatch/#early (need console inspect using chrome:inspect)
(3) Look in console log.

What is the expected result?
No error. Should say "testing early resolved".

What happens instead?
"testing early rejected"
"DOMException: The prompt() method may only be called once, following preventDefault()."

In other words, getting a beforeinstallprompt event and calling prompt() should be legal without first calling preventDefault(). In the current implementation it throws an InvalidStateError. This is a change from our previous design.

Rationale:
1. The spec doesn't say you have to call preventDefault before calling prompt: https://www.w3.org/TR/appmanifest/#prompt-method
2. With plans to make it not automatically prompt but still let you show a prompt by calling BeforeInstallPromptEvent.prompt() ( Issue 730991 ), it doesn't make sense to force you to call preventDefault before prompt, because the default action is not necessarily to show a prompt. If you want to show a prompt immediately, you should be able to just call prompt().

This should be very easy, just removing a check here:
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/modules/app_banner/BeforeInstallPromptEvent.cpp?l=76