Fenced frame: Use config to load in performance benchmark tests.

Fenced frames no longer support loading through the "src" attribute.
This CL updates the performance benchmarking tests to create fenced
frames using by setting the "config" attribute to a FencedFrameConfig.
This matches how other fenced frames tests handle loading.

Change-Id: I1802e8aad66b2cca4394a278546252b4bc72c837
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4828364
Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: Liam Brady <lbrady@google.com>
Cr-Commit-Position: refs/heads/main@{#1190367}