Do not expose DocumentLoader's request and original request

Once we stop using WebURLLoader for navigations, there would be
no Request in DocumentLoader. This patch removes some dependencies
on it from outside of DocumentLoader.

There are still some dependencies through WebDocumentLoader - these
will go later.

TBR=mmenke@chromium.org,rdevlin.cronin@chromium.org

Bug: 855189
Change-Id: I0c44d7627804286ccce3a6eac14fb7cd92321411
Reviewed-on: https://chromium-review.googlesource.com/c/1382506
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Nate Chapin <japhet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#617992}
18 files changed