[MPArch] Remove deprecated func usage in a SitePerProcessWebViewTest

This CL re-expresses the SitePerProcessWebViewTest.Shim_BlankWebview test
in terms of RenderFrameHost to avoid using deprecated function
DeprecatedWaitForSingleGuestCreated() in favor of its RenderFrameHost
version.

Bug: 1261928
Change-Id: I13786fb16f3af8a80fec83745fa109e85b8088c2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3861333


Commit-Queue: Antia Puentes <apuentes@igalia.com>
Reviewed-by: Kevin McNee <mcnee@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1040975}