[heap] Evacuator: Factor out dependencies

BUG=chromium:651354

Change-Id: Ic4cc354160b84267a4d930734120b68c2b7ba092
Reviewed-on: https://chromium-review.googlesource.com/458351Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44030}