Command-B and Command-I should work in contenteditable

All key events with the command key were marked as isSystemKey which
prevents Blink from handling them.

Command-B and Command-I are OS X standard key bindings but OS X doesn't handle them and we don't have them
in the menubar so we need to handle them inside Blink.

I also removed some unused lines from keyDownEntries in EditorClientImpl.cpp.

BUG=308680

Review URL: https://codereview.chromium.org/39073002

git-svn-id: svn://svn.chromium.org/blink/trunk@161459 bbb929c8-8fbe-4397-9dbb-9b2b20218538