Use reference of Frame rather than pointer in SpellCherker

This patch changes SpellChecker class to use reference of Frame as member variable |m_frame| instead of pointer to Frame to improve readability and ease of maintenance; no more null check. This patch also removes unused member function SpellChecker::frame().

This is follow up of a suggestion in
https://codereview.chromium.org/26565013/

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

Review URL: https://codereview.chromium.org/26674003

git-svn-id: svn://svn.chromium.org/blink/trunk@159410 bbb929c8-8fbe-4397-9dbb-9b2b20218538