Revert 161459 "Command-B and Command-I should work in contentedi..."

Broke compile on Linux/Aura.

> 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

TBR=keishi@chromium.org

Review URL: https://codereview.chromium.org/62633002

git-svn-id: svn://svn.chromium.org/blink/trunk@161460 bbb929c8-8fbe-4397-9dbb-9b2b20218538