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

This is a reland of commit 63ebca85

The GuestView PDF version of PrerenderWithCrossSiteEmbeddedPdf is flaky
on Mac 11 trybots when adding a TestNavigationObserver that waits for 2
navigations. Instead, keep the GuestView PDF version the same in the
test, and only have the OOPIF PDF codepath use the
TestNavigationObserver.

Original change's description:
> [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}

Bug: 324636880
Change-Id: I8570b01faab5b94a821e4754cdd3971da29fe5ad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5601699


Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1312364}