Move DocumentMarkerListEditor to its own .h/.cpp files

Based on Step 2 of yosin's plan:
https://codereview.chromium.org/2812423002#msg7

This is a follow-up to this CL where the DocumentMarkerListEditor class
was created inside DocumentMarkerController:
https://codereview.chromium.org/2812423002

BUG=707867

Review-Url: https://codereview.chromium.org/2818873002
Cr-Commit-Position: refs/heads/master@{#466224}
5 files changed