[OOPIF PDF] Remove PDF usage from ScriptingAPITest.NestedWebContents

ScriptingAPITest.NestedWebContents uses GuestView PDF to examine nested
WebContents behavior. Since OOPIF PDF will no longer have an inner
WebContents, make the test use a different way of creating a nested
WebContents.

Bug: 324637148
Change-Id: Ibe3648a5d7c2873c7a8edced0b3185abf35e57a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5398700


Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Commit-Queue: Andy Phan <andyphan@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1279210}