- const return type on Full{,Maybe,Heap}ObjectSlot::operator*
- Removed stale friend declarations
- Moved a method to the right section

Bug: v8:8557
Change-Id: I0bd3efdb96a7d997b4404d3a247bea55b083d611
Reviewed-on: https://chromium-review.googlesource.com/c/1467781Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59546}