fix(chips): adds aria-hidden to remove buttons

Closes #2345

Note:  That bug number was very satisfying to type.

src/components/chips/js/chipRemoveDirective.js

@@ -45,7 +45,7 @@
       // Child elements aren't available until after a $timeout tick as they are hidden by an
       // `ng-if`. see http://goo.gl/zIWfuw
       $timeout(function() {
-        element.attr('tabindex', '-1');
+        element.attr({ tabindex: -1, ariaHidden: true });
         element.find('button').attr('tabindex', '-1');
       });
     }

src/components/chips/js/chipsDirective.js

@@ -100,6 +100,7 @@
             ng-class="{\'md-focused\': $mdChipsCtrl.selectedChip == $index}">\
           <div class="md-chip-content"\
               tabindex="-1"\
+              aria-hidden="true"\
               ng-click="!$mdChipsCtrl.readonly && $mdChipsCtrl.selectChip($index)"\
               md-chip-transclude="$mdChipsCtrl.chipContentsTemplate"></div>\
           <div class="md-chip-remove-container"\
@@ -127,6 +128,7 @@
           class="md-chip-remove"\
           ng-if="!$mdChipsCtrl.readonly"\
           ng-click="$mdChipsCtrl.removeChipAndFocusInput($$replacedScope.$index)"\
+          aria-hidden="true"\
           tabindex="-1">\
         <md-icon md-svg-icon="close"></md-icon>\
         <span class="md-visually-hidden">\