Split the JSONConsumer into a base case and subclass so that we can
implement different consumers without needing to repeat the code that
deals with the finished semaphore and endpoint setters/getters.

This makes it possible to add a test consumer that just holds the trace
events in memory in a later CL.

This CL re-uses the existing perfetto-consumer.h file to some extent,
but it was accidentally leftover from a previous CL where we stopped
using it and switched to the JSON consumer instead.

Cq-Include-Trybots: luci.v8.try:v8_linux64_perfetto_dbg_ng
Bug: v8:8339
Change-Id: I56c305833c8937da76a8e6defbe7a52c59b25106
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1621938
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61799}