Adding tests for the prompt() method on the beforeinstallprompt event
Adding tests for the prompt() method on the beforeinstallprompt event

The prompt() method may only be called a single time following a
preventDefault() on the beforeinstallprompt event. Other cases result in
the method returning a rejected promise. These tests verify that prompt()
correctly allows the event to continue, whilst correctly returning a
rejected promise in other circumstances.

BUG=490545
R=mlamouri@chromium.org

Review URL: https://codereview.chromium.org/1202043002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@197847 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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