Update the ResourceRequest in DocumentLoader after the embedder modified it

The call on resource will end up calling DocumentLoader::willSendRequest
which stores the request. If we do this before the call on the host
(which will end up calling RenderFrameImpl::willSendRequest),
modifications by the host are not reflected in DocumentLoader::m_request.

BUG=331941
R=japhet@chromium.org

Review URL: https://codereview.chromium.org/126753002

git-svn-id: svn://svn.chromium.org/blink/trunk@164696 bbb929c8-8fbe-4397-9dbb-9b2b20218538