Implement the It2Me native messaging host.

1. Implemented It2MeNativeMessagingHost and its unit test.
2. It2MeHost now stores only a raw pointer to the host context object whose lifetime is managed by the creator of the it2me host (NPAPI plugin or it2me native messaging host).
3. The native messaging channel now stores the send_message callback instead of getting it from the caller every time a message is received.
4. The native messaging channel no longer tracks the number of pending requests.

BUG=309844

Review URL: https://codereview.chromium.org/61973004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237665 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237665