Added probe to determine if data reduction proxy can be used

Chrome will now probe the URL /connect, which is hosted by the
data reduction proxy, before using the proxy. This CL also
upstreams some of the settings logic for the proxy.

BUG=270958

Review URL: https://codereview.chromium.org/23458016

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227410 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227410