Content-shell support for the SetRPHRegistrationMode webdriver command

ChromeDriver implements the SetRPHRegistrationMode but the testing bots
use the content-shell instead of a Chrome instance. Hence, we need to
implement the support for this command in the testdriver-vendor.js via
the TestRunner APIs.

Bug: 1359103
Change-Id: Id0fd360ef487b6437e1fe5a104840500802cad12
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5205730


Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: Javier Fernandez <jfernandez@igalia.com>
Cr-Commit-Position: refs/heads/main@{#1261541}