Rename DocumentMarkerController::RemoveMarkers() to RemoveMarkersInternal()

yosin pointed out that it's confusing to have so many methods named
RemoveMarkers():
https://codereview.chromium.org/2820633002#msg30

This is the second of several renaming CLs I'm going to upload

BUG=707867

Review-Url: https://codereview.chromium.org/2829723006
Cr-Commit-Position: refs/heads/master@{#466309}
2 files changed