[Presentation API] Reject PresentationRequest.start() with the correct error.

This fixes the PresentationService so that start will reject with an
OperationError if there is a pending start Promise from the same frame.
This matches step 3 of the Presentation API spec [1].

Tested manually with the corresponding WPT [2].

[1] https://w3c.github.io/presentation-api/#selecting-a-presentation-display
[2] https://github.com/web-platform-tests/wpt/blob/master/presentation-api/controlling-ua/startNewPresentation_unsettledpromise-manual.https.html

Bug: 647504
Change-Id: If3f50e70321136556ed1a3d3ab1f4b763a6aa9eb
Reviewed-on: https://chromium-review.googlesource.com/c/1460101
Commit-Queue: mark a. foltz <mfoltz@chromium.org>
Reviewed-by: Takumi Fujimoto <takumif@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630975}
1 file changed