[MD settings] adding check for this.$ to C++ callback

[MD settings] adding check for this.$ to C++ callback

This adds a safety check in a JS call that may be called
from C++.  This comment/CL has more information about
why this is being done:
https://codereview.chromium.org/1365753003/#msg10

BUG=532739
Committed: https://crrev.com/cbe4835445c6a305b8f773ad65b5d87c253727b1
Cr-Commit-Position: refs/heads/master@{#351944}

[dschuyler, 2015-09-28 19:17:15]

dschuyler@chromium.org changed reviewers:
+ dbeam@chromium.org

[Dan Beam, 2015-09-28 20:11:48]

lgtm

[dschuyler, 2015-09-28 20:55:42]

The CQ bit was checked by dschuyler@chromium.org

[commit-bot: I haz the power, 2015-09-28 20:58:53]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1372093003/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1372093003/20001

[commit-bot: I haz the power, 2015-09-28 20:58:55]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-09-28 20:58:55]

CLs for remote refs other than refs/pending/heads/master must contain NOTRY=true
and NOPRESUBMIT=true in order for the CQ to process them

[dschuyler, 2015-10-02 02:04:19]

The CQ bit was checked by dschuyler@chromium.org

[dschuyler, 2015-10-02 02:04:19]

The patchset sent to the CQ was uploaded after l-g-t-m from dbeam@chromium.org
Link to the patchset: https://codereview.chromium.org/1372093003/#ps40001
(title: "merge with master")

[commit-bot: I haz the power, 2015-10-02 02:05:43]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1372093003/40001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1372093003/40001

[commit-bot: I haz the power, 2015-10-02 02:52:12]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2015-10-02 02:53:04]

Patchset 3 (id:??) landed as
https://crrev.com/cbe4835445c6a305b8f773ad65b5d87c253727b1
Cr-Commit-Position: refs/heads/master@{#351944}