Moved code from the .cc file to the .h file, and added comments on important
methods.

There is still room for more cleanup / refactor, but it doesn't seem worth
it right now.

Bug: v8:9396
Change-Id: Id14d3ccaa853e0704732d468df504c379cd114b2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1748735
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63205}