perf(sidenav): avoid extra repaints while scrolling (#2156)

Avoids repaints while scrolling inside of a sidenav. This isn't an issue on desktop browsers, but can potentially be on mobile. This can be checked by going to the dev tools > console fold-out menu > Rendering > Scrolling Performance Issues.

src/lib/sidenav/sidenav.scss

@@ -140,6 +140,9 @@ md-sidenav {
     box-sizing: border-box;
     height: 100%;
     overflow-y: auto; // TODO(kara): revisit scrolling behavior for sidenavs
+
+    // Prevents unnecessary repaints while scrolling.
+    transform: translateZ(0);
   }
 }