AppKit: CPScreen, CPColor PLATFORM conditional removal

- CPScreen.j and CPColor.j: one PLATFORM block each, resolved to
  runtime CPDOMAvailable guards; no preprocessor directives remain
  in either file
- work-in-progress towards complete removal of CPP directives

AppKit/CPColor.j

@@ -563,9 +563,10 @@ var cachedBlackColor,
 {
     // Fix to avoid a problem when compiling a theme (missing the alpha component as theme compiler doesn't have CSS rgba capability)
     var hasAlpha = YES;
-#if PLATFORM(DOM)
-    hasAlpha = CPFeatureIsCompatible(CPCSSRGBAFeature) && _components[3] != 1.0;
-#endif
+    if (CPDOMAvailable)
+    {
+        hasAlpha = CPFeatureIsCompatible(CPCSSRGBAFeature) && _components[3] != 1.0;
+    }
 
     _cssString = (hasAlpha ? "rgba(" : "rgb(") +
         parseInt(_components[0] * 255.0) + ", " +

AppKit/CPScreen.j

@@ -42,11 +42,10 @@
 */
 - (CGRect)visibleFrame
 {
-#if PLATFORM(DOM)
-    return CGRectMake(window.screen.availLeft, window.screen.availTop, window.screen.availWidth, window.screen.availHeight);
-#else
+    if (CPDOMAvailable)
+        return CGRectMake(window.screen.availLeft, window.screen.availTop, window.screen.availWidth, window.screen.availHeight);
+
     return CGRectMakeZero();
-#endif
 }
 
 @end