fix(backdrop): define var in valid scope for all reference sites

Closes #9226

src/components/backdrop/backdrop.js

@@ -31,13 +31,14 @@ angular
       // backdrop may be outside the $rootElement, tell ngAnimate to animate regardless
       if ($animate.pin) $animate.pin(element, $rootElement);
 
+      var bodyRect;
       $$rAF(function () {
         // If body scrolling has been disabled using mdUtil.disableBodyScroll(),
         // adjust the 'backdrop' height to account for the fixed 'body' top offset.
         // Note that this can be pretty expensive and is better done inside the $$rAF.
-        var body = $window.getComputedStyle($document[0].body);
-        if (body.position == 'fixed') {
-          var hViewport = parseInt(body.height, 10) + Math.abs(parseInt(body.top, 10));
+        bodyRect = $window.getComputedStyle($document[0].body);
+        if (bodyRect.position == 'fixed') {
+          var hViewport = parseInt(bodyRect.height, 10) + Math.abs(parseInt(bodyRect.top, 10));
           element.css({
             height: hViewport + 'px'
           });
@@ -66,7 +67,7 @@ angular
       });
 
       function resize() {
-        var hViewport = parseInt(body.height, 10) + Math.abs(parseInt(body.top, 10));
+        var hViewport = parseInt(bodyRect.height, 10) + Math.abs(parseInt(bodyRect.top, 10));
         element.css({
           height: hViewport + 'px'
         });