Use reference of Frame rather than pointer in Editor

This patch changes Editor 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 removed unused base class FrameDestructionObserver from Editor.

This patch is similar to https://codereview.chromium.org/26674003/ for SpellChecker.

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

Review URL: https://codereview.chromium.org/26277011

git-svn-id: svn://svn.chromium.org/blink/trunk@159419 bbb929c8-8fbe-4397-9dbb-9b2b20218538