DocumentLoader cleanup (mostly request-related)

Remove m_originalRequestCopy, which is nearly identical to m_originalRequest.
The only difference is that m_originalRequestCopy's url changes during a same-
document navigation. The only place that might differentiate between the two
is RenderViewImpl::UpdateURL(), which falls back to the original request url
when there is no redirect chain to use for the original url. However,
DocumentLoader::updateForSameDocumentNavigation() guarantees that there is a
non-empty redirect chain in the only case where the original request url
changes.

Remove some unusued and unimplemented DocumentLoader functions, and make
frameLoader() private.

Unify requestURL() and url(), and remove the non-const DocumentLoader::request
accessor, which isn't used.

BUG=none

Review URL: https://codereview.chromium.org/132673003

git-svn-id: svn://svn.chromium.org/blink/trunk@164828 bbb929c8-8fbe-4397-9dbb-9b2b20218538