Fix bug in setSize()

The function was referring to "cm", which is not necessarily defined.

Author: Dominator008

lib/codemirror.js

@@ -3130,7 +3130,7 @@ window.CodeMirror = (function() {
       if (width != null) this.display.wrapper.style.width = interpret(width);
       if (height != null) this.display.wrapper.style.height = interpret(height);
       if (this.options.lineWrapping)
-        cm.display.measureLineCache.length = cm.display.measureLineCachePos = 0;
+        this.display.measureLineCache.length = this.display.measureLineCachePos = 0;
       this.curOp.forceUpdate = true;
     }),