Add PushMessagingHasPermission runtime flag to expose PushManager.hasPermission()

Add PushMessagingHasPermission runtime flag to expose PushManager.hasPermission()

We want to keep hasPermission() away from the Push API for now. The
Permissions API will be used to fulfil that role. However, it is not
ready yet so in the mean time, we want to still be able to test the
feature until we move to the new version.

This is part of a three-sided CL:
1- <this>
2- https://codereview.chromium.org/937563004
3- https://codereview.chromium.org/936873002

BUG=449178
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=190471

[mlamouri (slow - plz ping), 2015-02-18 18:55:40]

mlamouri@chromium.org changed reviewers:
+ mkwst@chromium.org

[Peter Beverloo, 2015-02-18 19:02:59]

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

[Peter Beverloo, 2015-02-18 19:02:59]

push lgtm. let's hope we can get rid of this code soon :-)

[mlamouri (slow - plz ping), 2015-02-18 19:51:29]

New patchsets have been uploaded after l-g-t-m from peter@chromium.org

[Michael van Ouwerkerk, 2015-02-19 10:26:36]

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

[Michael van Ouwerkerk, 2015-02-19 10:26:36]

drive-by lgtm

[Mike West, 2015-02-19 10:34:33]

LGTM.

[mlamouri (slow - plz ping), 2015-02-19 10:44:06]

The CQ bit was checked by mlamouri@chromium.org

[mlamouri (slow - plz ping), 2015-02-19 11:20:55]

The CQ bit was unchecked by mlamouri@chromium.org

[mlamouri (slow - plz ping), 2015-02-19 11:20:58]

The CQ bit was checked by mlamouri@chromium.org

[commit-bot: I haz the power, 2015-02-19 12:09:45]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/926793004/40001

[commit-bot: I haz the power, 2015-02-19 14:05:06]

Committed patchset #3 (id:40001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=190471