Make sure the system request context specifies a user agent.
Make sure the system request context specifies a user agent.

BUG=76833
TEST=open about:net-internals. Change your proxy settings to use an http:// URL for PAC. Check that the request headers sent contains a User-Agent line.

Review URL: http://codereview.chromium.org/6732048

git-svn-id: http://src.chromium.org/svn/trunk/src@79371 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 1d22cc7b1a4196f0859bd2714d9a16a5864b415b
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser