Pepper: Move DoPostMessage out of trusted plugin.

This is the first effort at moving some of the ReverseService logic out of the
trusted plugin. I'd like to move these over soon since the tangled callback is
pretty nasty, and hidehiko@ has run into the ugliness while adding features for
Bare Metal Mode.

This one is pretty easy since it's stateless.

BUG=239656

Review URL: https://codereview.chromium.org/268793002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@269480 0039d316-1c4b-4281-b951-d872f2087c98
11 files changed