Added key binding for the overtype mode
Added key binding for the overtype mode

Set up the Insert key to trigger the OverWrite editing command and thus enable/disable the overtype mode.

Review URL: https://chromiumcodereview.appspot.com/14842018

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@150906 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs