[DMC #4] Add SingletonMarkerListImpl

Add SingletonMarkerListImpl

This marker list implementation can be used for MarkerType where we never
have more than one at a time. Right now, Composition is such a MarkerType. Once
I add SuggestionBackgroundHighlight, that will be another type that can use
this.

I've removed a few test cases that tested behavior that's no longer valid
(adding multiple composition markers, trying to get the rendered rects for
composition markers).

BUG=707867

[rlanday, 2017-04-18 08:15:11]

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

[rlanday, 2017-04-18 08:21:24]

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

[commit-bot: I haz the power, 2017-04-18 08:21:42]

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

[commit-bot: I haz the power, 2017-04-18 08:53:28]

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

[commit-bot: I haz the power, 2017-04-18 08:53:29]

Dry run: Try jobs failed on following builders:
  linux_chromium_tsan_rel_ng on master.tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)