Refactor DocumentMarkerController::MoveMarkers()

As we (yosin, xiaochengh, and I) discussed, I'm splitting the implementation
changes for this method I previously was including in
https://codereview.chromium.org/2812423002 into a separate CL.

We want to separate out the implementation changes we need to pull out the main
part of this method into a method on DocumentMarkerListEditor, and actually
adding that method.

This CL needs the AddMarker() method I'm adding in that CL, so I've removed the
MoveMarkers() changes entirely from that one and will upload a third CL that
adds DocumentMarkerListEditor::MoveMarkers().

BUG=707867

Review-Url: https://codereview.chromium.org/2819173002
Cr-Commit-Position: refs/heads/master@{#465124}
1 file changed