Move SpellCheckMarkerListImpl::RemoveMarkersUnderWords() impl to correct place

This method was moved from DocumentMarkerListEditor to SpellCheckMarkerListImpl
in https://codereview.chromium.org/2871823002, but the was left in
DocumentMarkerListEditor.cpp to make that CL smaller. This CL moves this method
to its final place in SpellCheckMarkerListImpl.cpp.

BUG=707867

Review-Url: https://codereview.chromium.org/2864243003
Cr-Commit-Position: refs/heads/master@{#472692}
2 files changed