Bug 868742 - Use scroll(Left|Top)Max to detect scrollable elements. r=mfinkle

mobile/android/chrome/content/browser.js

@@ -4388,8 +4388,7 @@ var BrowserEventHandler = {
        * - It's a select element showing multiple rows
        */
       if (checkElem) {
-        if (((elem.scrollHeight > elem.clientHeight) ||
-             (elem.scrollWidth > elem.clientWidth)) &&
+        if ((elem.scrollTopMax > 0 || elem.scrollLeftMax > 0) &&
             (this._hasScrollableOverflow(elem) ||
              elem.mozMatchesSelector("html, body, textarea")) ||
             (elem instanceof HTMLSelectElement && (elem.size > 1 || elem.multiple))) {
@@ -4420,10 +4419,10 @@ var BrowserEventHandler = {
 
   _elementCanScroll: function(elem, x, y) {
     let scrollX = (x < 0 && elem.scrollLeft > 0)
-               || (x > 0 && elem.scrollLeft < (elem.scrollWidth - elem.clientWidth));
+               || (x > 0 && elem.scrollLeft < elem.scrollLeftMax);
 
     let scrollY = (y < 0 && elem.scrollTop > 0)
-               || (y > 0 && elem.scrollTop < (elem.scrollHeight - elem.clientHeight));
+               || (y > 0 && elem.scrollTop < elem.scrollTopMax);
 
     return scrollX || scrollY;
   }