Pesto: Prevent overscrolling on recipe page. (#5237)

BUG=https://github.com/flutter/flutter/issues/5098

examples/flutter_gallery/lib/demo/pesto_demo.dart

@@ -345,7 +345,9 @@ class _RecipePageState extends State<_RecipePage> {
             )
           )
         ),
-        new ScrollableViewport(
+        new ClampOverscrolls(
+          value: true,
+          child: new ScrollableViewport(
             child: new RepaintBoundary(
               child: new Padding(
                 padding: new EdgeInsets.only(top: appBarHeight),
@@ -370,6 +372,7 @@ class _RecipePageState extends State<_RecipePage> {
               )
             )
           )
+        )
       ]
     );
   }