This patch works around some problems when CORS requests redirect.

- Instead of allowing the request to continue, DocumentThreadableLoader (DTL)
cancels the old request and starts a new one. However, the chromium embedding
layer gives us a new request without the original headers. In order to create a new
request that matches the old one, DTL now stores the request headers that are
allowed by the CORS redirect steps, and in case of a cross origin redirect, copies
them to the new request.

- Remove calls to clearHTTPAccept and clearHTTPContentType when CORS redirects.

- Eliminates these methods on ResourceRequest, since they are now unused.

BUG=162183

Review URL: https://codereview.chromium.org/130063002

git-svn-id: svn://svn.chromium.org/blink/trunk@172648 bbb929c8-8fbe-4397-9dbb-9b2b20218538