Reset prior ChannelSendFrameTransformerDelegate on re-assignment

When setting a frame transformer on ChannelSend via
InitFrameTransformerDelegate, any existing frame_transformer_delegate_
was overwritten without calling Reset() on the prior delegate.

The orphaned delegate remained registered with its transformer and
retained a callback lambda capturing a raw ChannelSend pointer. Upon
destruction of ChannelSend, only the active delegate was reset,
leaving the orphaned delegate with a dangling pointer that could be
invoked post-teardown.

This change calls Reset() on any existing frame_transformer_delegate_
before replacing it with a new delegate, unregistering the old
delegate and disarming its callback lambda.

Bug: b:510386389
Change-Id: Ib0cf86c129280551daaeea3fcd5c08b9c5172b1b
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/487001
Reviewed-by: Per Åhgren <peah@webrtc.org>
Reviewed-by: Jakob Ivarsson‎ <jakobi@webrtc.org>
Commit-Queue: Per Åhgren <peah@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48131}