Fenced frames: Fix automatic beacon dev tool support

`SendFencedFrameReportingBeaconInternal` used to take an argument
`initiator_frame_tree_node_id`, used only for dev tools support. This
argument was set incorrectly for automatic beacons (target frame rather
than initiator frame). This CL fixes the bug by eliminating the argument
entirely; the function is always called on the initiator RFHI, so we can
just get the current FTN id inside the function.

Change-Id: Ic9403cc517c15f4022fd84d1d1ab67bc2c6ac1df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4956158
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Liam Brady <lbrady@google.com>
Commit-Queue: Garrett Tanzer <gtanzer@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1212435}