Don't create RenderedDocumentMarkers in SpellCheckMarkerListImpl

I meant to do this as part of https://codereview.chromium.org/2883503004 but
forgot that we have a custom Add() method in SpellCheckMarkerListImpl. Oops

BUG=707867

Review-Url: https://codereview.chromium.org/2899413003
Cr-Commit-Position: refs/heads/master@{#474607}
1 file changed