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Native Messaging API is useful only on desktop OSes, i.e. on Linux, OSX and 
Windows. Disable it in all other configurations. Also improved how we handle 
the case when native messaging is disabled is handled: now the app will get 
disconnect notification.

BUG=142915
TBR=thakis@chromium.org


Review URL: https://chromiumcodereview.appspot.com/12218099

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@182197 0039d316-1c4b-4281-b951-d872f2087c98