Move SpellCheckMarkerListImpl::Add() implementation from DMLEditor.cpp

Move SpellCheckMarkerListImpl::Add() from DocumentMarkerListEditor.cpp to
SpellCheckMarkerListImpl.cpp. This method implementation was left in
DocumentMarkerListEditor in https://codereview.chromium.org/2829543002
to make that patch smaller. This patch moves it to its final location.

BUG=707867

Review-Url: https://codereview.chromium.org/2851633003
Cr-Commit-Position: refs/heads/master@{#471229}
2 files changed