It2me native messaging host: webapp implementation.

The webapp implementation of the it2me native messaging is based on the me2me couterpart. But I got rid of the per-message onDone/onError callbacks because unlike me2me, all requests to the it2me host are asynchronous: direct responses to all requests are meaningless. Only the asynchronous callbacks on host state change and nat policy update need to be processed.

I verified that this is fully functional on Linux. Windows and Mac verification haven't been done and are planned after the linux work is completed.

BUG=309844

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=248072

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=248157

Review URL: https://codereview.chromium.org/138503009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248255 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248255