Add a clearHTTPHeaderField method to ResourceResponse and ResourceRequest.

This addresses two nearly identical FIXMEs.

BUG=None
TEST=None
R=abarth@chromium.org,darin@chromium.org

Review URL: https://chromiumcodereview.appspot.com/24873003

git-svn-id: svn://svn.chromium.org/blink/trunk@158410 bbb929c8-8fbe-4397-9dbb-9b2b20218538