Issue 6857: Give the ability to modify the returned value

Steps to reproduce

  1. Open Firebug (F12) and Enable the Script and Console panels
  2. Select the Script panel and create a breakpoint at line 17
  3. Click this button to stop JS execution at line 17
  4. Step over once (F10) and check out the Watch panel. It now displays <return value>
  5. Double click on the value (number 100) and change it.
  6. Resume the debugger (F8) and check out the Console panel. The new value is logged.