Move spell checker related API implementation from WebLocalFrameImpl to SpellChecker

This patch moves spell checker related API implementation from
|WebLocalFrameImpl| class to |SpellChecker| class to make "web" as thin API
layer and ease of maintenance, e.g. we don't need to ask Web API owner for
reviewing implementation changes.

BUG=n/a
TEST=n/a; no behavior changes

Review URL: https://codereview.chromium.org/350133002

git-svn-id: svn://svn.chromium.org/blink/trunk@176823 bbb929c8-8fbe-4397-9dbb-9b2b20218538