Add type cast for SpellCheckMarkerListImpl

This will be used to cast a DocumentMarkerList* to SpellCheckMarkerListImpl* in
DocumentMarkerController once RemoveMarkersUnderWords() is a method defined on
SpellCheckMarkerListImpl instead of DocumentMarkerList.

BUG=707867

Review-Url: https://codereview.chromium.org/2873483002
Cr-Commit-Position: refs/heads/master@{#472324}
1 file changed