Introduce a hidden element as a reference for zoom computation

Rather than rely on the change in body position to compute the mobile
zoom compensation introduce a hidden, absolutely position element whose
offset can be measured.  The body element can be offset due to vertical
margin collapse even when the element itself is positioned at the
origin. When this happens the body offset measurements become unreliable.

src/utility.js

@@ -212,8 +212,20 @@ function countFlags(value) {
  * positioned at 0,0.
  */
 function activateChromeZoomedOffsetPatch() {
+	var style;
 	if (/Chrome\/[.0-9]*/.test(navigator.userAgent)) {
 		session.chromePatchRefElement = $(document.body);
+		style = {
+			top: 0,
+			left: 0,
+			position: 'absolute',
+			display: 'hidden',
+			height: '1px',
+			margin: 0,
+			width: '1px',
+			zIndex: -1
+		};
+		session.chromePatchRefElement = $('<div/>').css(style).appendTo($(document.body));
 	}
 }