fix(disableScroll): fix scroll mask z-index blocking backdrops

closes #3283, closes #3269, closes #3245

Author: rschmukler

src/components/backdrop/backdrop.scss

@@ -1,5 +1,8 @@
 md-backdrop {
   z-index: $z-index-backdrop;
+  &.md-menu-backdrop {
+    z-index: $z-index-menu - 1;
+  }
   &.md-select-backdrop {
     z-index: $z-index-dialog + 1;
   }

src/components/menu/menu-interim-element.js

@@ -127,7 +127,9 @@ function MenuProvider($$interimElementProvider) {
         opts.backdrop && opts.backdrop.on('click', function(e) {
           e.preventDefault();
           e.stopPropagation();
-          opts.mdMenuCtrl.close(true);
+          scope.$apply(function() {
+            opts.mdMenuCtrl.close(true);
+          });
         });
 
         // Wire up keyboard listeners.

src/core/util/util.js

@@ -66,7 +66,7 @@ angular.module('material.core')
         // Creates a virtual scrolling mask to absorb touchmove, keyboard, scrollbar clicking, and wheel events
         function disableElementScroll() {
           var zIndex = $window.getComputedStyle(element[0]).zIndex - 1;
-          if (isNaN(zIndex)) zIndex = 99;
+          if (isNaN(zIndex)) zIndex = 50;
           var scrollMask = angular.element(
             '<div class="md-scroll-mask" style="z-index: ' + zIndex + '">' +
             '  <div class="md-scroll-mask-bar"></div>' +