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: de016e52bd170d2d4f2344f9bf92d50478b649e0@79371 0039d316-1c4b-4281-b951-d872f2087c98

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