Move NativeMessagingHostMain to me2me_native_messaging_host_main.cc

The plan is to implement a DelegatingPairingRegistryDelegate class and use it in the normal (unelevated) native messaging host process. But before I work on that I want to do this small cleanup first.

BUG=325567

Review URL: https://codereview.chromium.org/100333003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238762 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238762