Clean up DocumentMarkerController::copyMarkers() (now it's moveMarkers())

This method is kind of weird since it leaves the markers in the source list but
sometimes changes one of them (if it spans a node boundary). In this CL, I'm
cleaning it up so it removes the markers from the source list after adding them
to the destination list.

BUG=707867

Review-Url: https://codereview.chromium.org/2810463002
Cr-Commit-Position: refs/heads/master@{#463509}
3 files changed