- do not strip User-Agent while flattening headers in renderer
- do not override User-Agent header in the network stack if one is already supplied by higher level;
- added RequestHeaders::SetHeaderIfMissing
TEST=URLRequestTestHTTP.OverrideUserAgent
BUG=http://crbug.com/67063
Review URL=http://codereview.chromium.org/6065008
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@72131 0039d316-1c4b-4281-b951-d872f2087c98