Get rid of redundant member function Editor::didBeginEditing()

This patch removes member function Editor::didBeginEditing(), which calls
SpellChecker::didBeginEditing() to reduce source code size of Editor class for
ease of maintenance.

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

Review URL: https://codereview.chromium.org/59153011

git-svn-id: svn://svn.chromium.org/blink/trunk@162421 bbb929c8-8fbe-4397-9dbb-9b2b20218538