Fenced frames: Remove FencedFrameConfig constructor in anonymous iframe

wpts.

Changed to use generateURNFromFledge as we do not want to expose
FencedFrameConfig constructor in web platform tests.

Bug: 1469414
Change-Id: If57afb4a275041a34829fdd81f7e9e8e503980db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4753832


Reviewed-by: Dominic Farolino <dom@chromium.org>
Commit-Queue: Xiaochen Zhou <xiaochenzh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#11804513}