[MPArch] Delete WebContents direct access in SitePerProcessWebViewTest

This CL re-expresses the SitePerProcessWebViewTest.ContentScript test
in terms of RenderFrameHost to avoid accessing WebContents directly.

Bug: 1261928
Change-Id: I0de5f7f3409664d7535fd37e294597a262717f7c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3862467


Commit-Queue: Antia Puentes <apuentes@igalia.com>
Reviewed-by: Kevin McNee <mcnee@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1040974}