Remove unnecessary call to Serialize.

Change a Handle cast to a Ref cast.

Add a CHECK to JSHeapBroker::SetFeedback that the source is valid.

Bug: v8:7790
Change-Id: I6a2f10d45456c6b128beff009016009457dc109c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1773248
Commit-Queue: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63421}