Added getting proxy config from Chrome and default proxy transport

This adds methods for getting the proxy settings from Chrome in either a
synchronous or asynchronous way. It also then adds a method to
http_transport for creating a Transport based off the proxy settings
rather than having to call into the curl specific methods.

BUG=None
TEST=Unit tests pass

Change-Id: I4f3df09740e4f3a78563fbb1987928bd794fd3c5
Reviewed-on: https://chromium-review.googlesource.com/687830
Commit-Ready: Jeffrey Kardatzke <jkardatzke@google.com>
Tested-by: Jeffrey Kardatzke <jkardatzke@google.com>
Reviewed-by: Dan Erat <derat@chromium.org>
7 files changed