Adding a word to dictionary should remove spelling markers.

This patch adds code for removing spelling markers under words on 
blink's side.

BUG=3506

Review URL: https://codereview.chromium.org/419563003

git-svn-id: svn://svn.chromium.org/blink/trunk@180382 bbb929c8-8fbe-4397-9dbb-9b2b20218538