[Presentation API] PresentationRequest::getAvailability() should return the same promise

Add a ScriptPromiseProperty to PresentationRequest to store PresentationAvailability promise object. getAvailability() will always return the same promise object.

BUG=653131

Review-Url: https://codereview.chromium.org/2572473003
Cr-Commit-Position: refs/heads/master@{#439269}
13 files changed