Add DocumentMarkerList implementation classes

Add DocumentMarkerList implementation classes

Step 3 ("Implement specialized DMLImpl classes with test cases") of my plan to
split up this refactor: https://codereview.chromium.org/2805553003#msg27

I haven't actually written the tests yet, but I wanted to put this up for review
so we can decide if we're on the right track. I can split this up for review but
I figured there's more context this way for discussing the design.

BUG=707867

[rlanday, 2017-04-12 01:37:37]

The CQ bit was checked by rlanday@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2017-04-12 01:38:28]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[rlanday, 2017-04-12 01:38:53]

The CQ bit was checked by rlanday@chromium.org to run a CQ dry run

[rlanday, 2017-04-12 01:39:02]

rlanday@chromium.org changed reviewers:
+ xiaochengh@chromium.org, yosin@chromium.org

[commit-bot: I haz the power, 2017-04-12 01:39:33]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[yosin_UTC9, 2017-04-12 01:52:35]

This CL is what I'm afraid of.
We don't want to migrate all DM list types one.
We want to migrate one by one on demand, e.g. we don't need to IME marker at
this time since
it works well in current implementation.

[commit-bot: I haz the power, 2017-04-12 03:28:55]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2017-04-12 03:28:56]

Dry run: This issue passed the CQ dry run.