[OOPIF PDF] Enable some PDF extension tests (11/n)

Enable PDFExtensionPrerenderTest.PrerenderWithCrossSiteEmbeddedPdf for
OOPIF PDF, by waiting for the PDF iframe navigation and waiting for the
PDF viewer to load.

Adjust the comments for
PDFExtensionPrerenderAndFencedFrameTest.LoadPDFInPrerender, since the
OOPIF PDF test version cannot be written until the bug is fixed.

Bug: 324636880
Change-Id: I27e086cf44bf47b5c588dfe772b3153c13f4faee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5585872


Commit-Queue: Andy Phan <andyphan@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1308726}