DevTools: [CodeMirror] Ctrl-Z removes all content

Pressing Ctrl-Z in the very beginning of editing clears
all content.

BUG=

Review URL: https://chromiumcodereview.appspot.com/13884010

git-svn-id: svn://svn.chromium.org/blink/trunk@148323 bbb929c8-8fbe-4397-9dbb-9b2b20218538