perf(dialog): avoid repaintin dialog content element on scroll

Fixes the `<md-dialog-content>` element being repainted on scroll.

Fixes #6878.

Author: crisbeto

src/lib/core/style/_vendor-prefixes.scss

@@ -33,4 +33,9 @@
   cursor: -webkit-grabbing;
   cursor: grabbing;
 }
+
+@mixin backface-visibility($value) {
+  -webkit-backface-visibility: $value;
+  backface-visibility: $value;
+}
 /* stylelint-enable */

src/lib/dialog/dialog.scss

@@ -1,5 +1,6 @@
 @import '../core/style/elevation';
 @import '../core/a11y/a11y';
+@import '../core/a11y/vendor-prefixes';
 
 
 $mat-dialog-padding: 24px !default;
@@ -35,6 +36,9 @@ $mat-dialog-button-margin: 8px !default;
   max-height: $mat-dialog-max-height;
   overflow: auto;
   -webkit-overflow-scrolling: touch;
+
+  // Promote the content to a new GPU layer to avoid repaints on scroll.
+  @include backface-visibility($hidden);
 }
 
 .mat-dialog-title {