[OOPIF PDF] Introduce and use test method for loading embedded PDFs

Introduce and use PDFExtensionTestBase::LoadPdfInFirstChild() and
PDFExtensionTestBase::LoadPdfInFirstChildGetExtensionHost(), which are
test helper methods for loading PDFs in HTML pages embedding PDF in the
first child frame. This reduces the amount of repeated code for
navigating to the URL and ensuring the PDF load.

Bug: 324636880
Change-Id: I3ab17425cd7c8f7a4e4d7794d5fd4a77f0e35edf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5328703
Commit-Queue: Andy Phan <andyphan@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1266586}