- Make FeedbackVector backing store a WeakFixedArray.
- "feedback" is always strong but "extra" might be weak.
- Whenever the handler stored in FeedbackVector is a WeakCell to a transition
  Map, replace it with an in-place weak reference.
For a more detailed description of the changes, see the design doc

https://docs.google.com/document/d/1P8cIme2wKszdYt64ObAiuh6pXgLnrrn80Hpl1ejJbOU/edit#heading=h.ijx1oculrikp

BUG=v8:7308

Change-Id: I72c5cf6597ef24d4c22a1fe8e25b67ca196d4ec8
Reviewed-on: https://chromium-review.googlesource.com/1027855
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53051}