Update PushEvent and PushMessageData to match the spec.

Update PushEvent and PushMessageData to match the spec.

The specification defines that PushEvents can be initialized with a
data member that is of type PushMessageDataInit, rather than an actual
PushMessageData object. This allows us to remove the constructor from
the PushMessageData object entirely, aligning with the specification.

https://w3c.github.io/push-api/#pushmessagedata-interface

This CL also implements the following pull requests, which defines the
PushMessageDataInit type to also encapsulate ArrayBuffer(View)?.

https://github.com/w3c/push-api/pull/160

Note that the PushMessageData object has not shipped to the world yet.

R=mvanouwerkerk
BUG=

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=201129

[Peter Beverloo, 2015-08-24 17:23:35]

PTAL

[Michael van Ouwerkerk, 2015-08-25 11:11:27]

mvanouwerkerk@chromium.org changed reviewers:
+ mvanouwerkerk@chromium.org

[Michael van Ouwerkerk, 2015-08-25 11:11:28]

lgtm Thanks!

[Peter Beverloo, 2015-08-25 12:04:17]

The CQ bit was checked by peter@chromium.org

[commit-bot: I haz the power, 2015-08-25 12:04:20]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1311853002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1311853002/1

[commit-bot: I haz the power, 2015-08-25 12:59:57]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=201129