Fix crash in shadowDOM fenced frames before window is attached.

Frame::Parent can get called before the DomWindow is attached. Use
the FencedFramesEnabledByRuntimeFlag because it is consistent with
the other blink::features::IsFencedFrameEnabled public API checks.

BUG=1322862

Change-Id: I64b90a331b5f64059343d98b5dc3cad49503d5c7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3629959


Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Kevin McNee <mcnee@chromium.org>
Cr-Commit-Position: refs/heads/main@{#999900}