Move editor keyboard handling code to Editor from EditorClientImpl

This patch moves editor key bindings handling to Editor class from EditorClientImpl class to make API layer thin for ease of maintenance.

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

Review URL: https://codereview.chromium.org/75033002

git-svn-id: svn://svn.chromium.org/blink/trunk@162451 bbb929c8-8fbe-4397-9dbb-9b2b20218538