Separate push event data from the rest of the W3C Push API flag.

Separate push event data from the rest of the W3C Push API flag.

Supporting payloads for incoming push messages is blocked on resolving
a number of security concerns, key in which is enforcing encryption as
part of the W3C Push API. Not exposing the "data" attribute and the
PushMessageData object for PushEvents is the best option for now, which
allows developers to feature detect support for payloads.

Availability of the payloads will be toggled by the embedder.

This is part of a three-sided patch:
  [1] This patch.
  [2] https://codereview.chromium.org/874613002
  [3] https://codereview.chromium.org/869303002

BUG=449184
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=188932

[Peter Beverloo, 2015-01-23 18:20:40]

peter@chromium.org changed reviewers:
+ mkwst@chromium.org, mvanouwerkerk@chromium.org

[Peter Beverloo, 2015-01-23 18:20:40]

+mvanouwerkerk for Push API
+mkwst for the runtime feature

[Michael van Ouwerkerk, 2015-01-23 18:23:44]

lgtm

[Mike West, 2015-01-23 19:03:21]

lgtm

[Peter Beverloo, 2015-01-24 18:17:08]

The CQ bit was checked by peter@chromium.org

[commit-bot: I haz the power, 2015-01-24 18:17:47]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/872013002/20001

[commit-bot: I haz the power, 2015-01-24 18:20:46]

Committed patchset #2 (id:20001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=188932