[MPArch] Avoid deprecated func in WebViewPdfTest

This CL rewrites test WebViewPdfTest.NestedGuestContainerBounds in
terms of RenderFrameHost to avoid the use of the deprecated function
DeprecatedGetGuestWebContentsList().

Bug: 1261928
Change-Id: I508cc4380a4854aa328a12f039d8432afa28c759
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3867401


Commit-Queue: Antia Puentes <apuentes@igalia.com>
Reviewed-by: Kevin McNee <mcnee@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1041707}