Add the --trace-feedback-updates flag (disabled by default, enabled by
the v8_enable_trace_feedback_updates gn arg), which traces updates to
feedback slots.

Change-Id: Ib8f02f958e2adf04abda5d4ed680e29fa04895ab
Reviewed-on: https://chromium-review.googlesource.com/725814Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48983}