[OOPIF PDF] Navigate during set up for PDFWebContentsHelperTest

Navigate in SetUpOnMainThread(), instead of at the start of tests.

Some tests need navigation to properly set the content::WebContents. For
OOPIF PDF, PDFWebContentsHelper will be changed to use DocumentUserData
instead. Because of this, the navigation needs to occur during
SetUpOnMainThread(), otherwise the PDFWebContentsHelper object will be
destroyed on navigation.

Also navigate to "about:blank" instead of an empty URL to silence a
warning.

Bug: 1445746
Change-Id: I3ce4e3ae9fb899e682e48cc1a0ebf58c7f09d346
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4798977
Reviewed-by: Lei Zhang <thestig@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Andy Phan <andyphan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1186123}