Reland: GC: Refactor public incremental marking interface in heap

Reland: GC: Refactor public incremental marking interface in heap

Combines:
* https://codereview.chromium.org/1273483002/
* https://codereview.chromium.org/1266793003/

This reverts commit 0215fb56f4c75b054116632039edbff0d7f40373.

BUG=

Committed: https://crrev.com/9d7ebcf7f508b987464575b481476ce0a49d98b1
Cr-Commit-Position: refs/heads/master@{#30014}

[Michael Lippautz, 2015-08-04 18:08:18]

mlippautz@chromium.org changed reviewers:
+ ulan@chromium.org

[ulan, 2015-08-04 18:20:20]

lgtm

[Michael Lippautz, 2015-08-04 18:29:18]

The CQ bit was checked by mlippautz@chromium.org

[commit-bot: I haz the power, 2015-08-04 18:29:27]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1265273002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1265273002/1

[commit-bot: I haz the power, 2015-08-04 19:01:05]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-04 19:01:27]

Patchset 1 (id:??) landed as
https://crrev.com/9d7ebcf7f508b987464575b481476ce0a49d98b1
Cr-Commit-Position: refs/heads/master@{#30014}