DocumentLoader: get rid of some usages of ResourceRequest

Request is going away on CommitNavigation path, so replace
request usage with specific fields we need.
Also, DocumentLoader will be loading body by itself, so
it needs to know about deferring now.

This corresponds to the step 6.e from the doc linked to the bug.

Bug: 855189
Change-Id: Id08ce34533728951e67b150e1974444d8fb86b2a
Reviewed-on: https://chromium-review.googlesource.com/c/1401054
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Nate Chapin <japhet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621852}
6 files changed