AppKit: first method-definition gates removed (Increment A)

- CPTextField, CPTokenField, CPTextView: the PLATFORM(DOM) gates
  around _inputElement (superclass and override) and _cumulativeOffset
  are removed; the methods now exist under both targets with runtime
  CPDOMAvailable guards returning nil (object methods) or a zero
  point (struct method) when no DOM is present
- contract change: under CommonJS these methods change from absent to
  present-and-inert; all callers are browser event paths never
  exercised by the theme blender or nib2cib
- zero preprocessor directives remain in these three files
- work-in-progress towards complete removal of CPP directives

AppKit/CPTextView/CPTextView.j

@@ -1031,9 +1031,10 @@ Sets the selection to a range of characters in response to user action.
        [_CPNativeInputManager focusForClipboardOfTextView:self];
 }
 
-#if PLATFORM(DOM)
 - (CGPoint)_cumulativeOffset
 {
+    if (!CPDOMAvailable) return CGPointMake(0, 0);
+
     var top = 0,
         left = 0,
         element = self._DOMElement;
@@ -1048,7 +1049,6 @@ Sets the selection to a range of characters in response to user action.
 
     return CGPointMake(left, top);
 }
-#endif
 
 - (CPArray)selectedRanges
 {