[Save to Drive] Call `OnDocumentLoadComplete` for embedded PDFs

This CL ensures that the "Save to Drive" feature works for embedded
PDFs. It does this by making sure the `OnDocumentLoadComplete` is called
when an embedded PDF finishes loading, just as it already does for a
full-page PDF.

Demo:
https://screencast.googleplex.com/cast/NTY5MjU4MTUwNDYxNDQwMHw5YTcxNGUwZi05Mw

Bug: 446881875
Change-Id: I8804c1692dd7fac33726d7f4f9a21ffe94a6b406
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6973665
Commit-Queue: Andy Phan <andyphan@chromium.org>
Reviewed-by: Andy Phan <andyphan@chromium.org>
Auto-Submit: Faizur Rahman <faizur@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1521520}