[MPArch] Remove WebContents access in WebViewTest.SelectShowHide

This CL removes WebContents access in
WebViewTest.SelectShowHide and uses directly RenderFrameHost in
order to hide the direct WebContents access.

Bug: 1261928
Change-Id: I772e47876c19ef839a57f7c2869b315c3a08dde9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3861258


Reviewed-by: Kevin McNee <mcnee@chromium.org>
Commit-Queue: Julie Jeongeun Kim <jkim@igalia.com>
Cr-Commit-Position: refs/heads/main@{#1040850}