Allow LocalServers to be created via telemetry API

This API allows a LocalServer subclass to be created by the user of the
telemetry API. The key challenge here is preventing the LocalServer subclasses
from getting access to the browser backend, which contains our private APIs.
This is done by giving the server access to a LocalServerController, which
internally does the priviliged calls out to the backend to set up port
forwarding.

R=tonyg
NOTRY=True

Review URL: https://codereview.chromium.org/133793009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248352 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248352