Reason for revert:
Revive left-trimming until new strategies are evaluated and decided on.

Partially reverted changes from:
https://codereview.chromium.org/1474203003/

BUG=v8:4606
LOG=N

Original issue's description:
> [heap] delete Heap::LeftTrimFixedAray
>
> No more uses left for this code (thank goodness.)
>
> R=hpayer@chromium.org
> BUG=
>
> Committed: https://crrev.com/dc3442b1ec803916eb6c1882ab4c3ec6fd44dd55
> Cr-Commit-Position: refs/heads/master@{#32305}

TBR=hpayer@chromium.org,ofrobots@google.com

Review URL: https://codereview.chromium.org/1535933002

Cr-Commit-Position: refs/heads/master@{#32965}