Add DocumentMarkerListEditor::MoveMarkers()

Follow-up to https://codereview.chromium.org/2812423002/ (add
DocumentMarkerListEditor class) and https://codereview.chromium.org/2819173002
(refactor DocumentMarkerController::MoveMarkers()): this CL moves some of the
DocumentMarkerController::MoveMarkers() code into a new
DocumentMarkerListEditor::MoveMarkers() method

BUG=707867

Review-Url: https://codereview.chromium.org/2820983002
Cr-Commit-Position: refs/heads/master@{#465894}
2 files changed