Rename SpellChcker class to SpellCheckRequester

This patch renames SpellChecker to SpellCheckRequesterto make class name more explicitly denotes what it does, and free its name to use name SpellChecker for decoupling spell checking and Editor class, and remove unnecessary #include directives.

This patch is a follow up patch of r155920.

BUG=n/a
TEST=n/a; no behavior changes
R=tkent@chromium.org

Review URL: https://codereview.chromium.org/22797002

git-svn-id: svn://svn.chromium.org/blink/trunk@156129 bbb929c8-8fbe-4397-9dbb-9b2b20218538