[heap] Separate scavenger functionality into own file.

[heap] Separate scavenger functionality into own file.

This moves scavenging functionality into a separate component so that
neither the scavenger nor objects-visiting need to be exposed outside
the heap.

R=hpayer@chromium.org,mlippautz@chromium.org

Review URL: https://codereview.chromium.org/1323993004

Cr-Commit-Position: refs/heads/master@{#30712}