jsoncpp: force alignment of Json::Value::null to avoid crashes on ARM.

BUG=109997,https://code.google.com/p/webrtc/issues/detail?id=1777

Review URL: https://codereview.chromium.org/24984004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226099 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226099