Prepare DocumentMarkerController for using different DocumentMarkerList impls

I'm going to start writing different implementations of DocumentMarkerList for
the different MarkerTypes. This CL prepares us to be able to easily specify the
marker list class we want to use for each MarkerType.

BUG=707867

Review-Url: https://codereview.chromium.org/2826493003
Cr-Commit-Position: refs/heads/master@{#467609}
1 file changed