Remove usage of GetProvisionalDocumentLoader from AppWindowCustomBindings

When resuming parser, we are guaranteed that DocumentLoader has been
already committed, since that happens synchronously.

Bug: 855189
Change-Id: I6904a3559aca34221a24f08f611c15c7997916c7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1589140
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657006}