Extract code to mark an object during incremental marking

Extract code to mark an object during incremental marking

Not only does this remove code duplication, I also plan to use this for
unifying reference group marking later

BUG=none
R=hpayer@chromium.org
LOG=n

Committed: https://crrev.com/a6a887d083286aa1996122979e02d09c042a90fe
Cr-Commit-Position: refs/heads/master@{#27180}

[Hannes Payer (out of office), 2015-03-13 12:04:50]

lgtm

[Hannes Payer (out of office), 2015-03-13 12:04:52]

lgtm

lgtm

[jochen (gone - plz use gerrit), 2015-03-13 12:05:31]

The CQ bit was checked by jochen@chromium.org

[commit-bot: I haz the power, 2015-03-13 12:05:37]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1007793002/1

[commit-bot: I haz the power, 2015-03-13 12:14:06]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-13 12:14:22]

Patchset 1 (id:??) landed as
https://crrev.com/a6a887d083286aa1996122979e02d09c042a90fe
Cr-Commit-Position: refs/heads/master@{#27180}