RN: Fix Drawing Rect for ReactScrollView

Summary:
Fixes `ReactScrollView` so that it respects the drawing rect (i.e. the bounding box of the element).

In JavaScript, this is the backing view for `ScrollView` (vertical) on Android.

Reviewed By: fadinghorse

Differential Revision: D8710256

fbshipit-source-id: f3bd96e39b8569cfcb21e486944b70fdb57c12b6

ReactAndroid/src/main/java/com/facebook/react/uimanager/ViewProps.java

@@ -102,6 +102,7 @@ public class ViewProps {
   public static final String OVERFLOW = "overflow";
 
   public static final String HIDDEN = "hidden";
+  public static final String SCROLL = "scroll";
   public static final String VISIBLE = "visible";
 
   public static final String ALLOW_FONT_SCALING = "allowFontScaling";

ReactAndroid/src/main/java/com/facebook/react/views/scroll/ReactScrollView.java

@@ -47,6 +47,7 @@ public class ReactScrollView extends ScrollView implements ReactClippingViewGrou
   private final OnScrollDispatchHelper mOnScrollDispatchHelper = new OnScrollDispatchHelper();
   private final @Nullable OverScroller mScroller;
   private final VelocityHelper mVelocityHelper = new VelocityHelper();
+  private final Rect mRect = new Rect(); // for reuse to avoid allocation
 
   private @Nullable Rect mClippingRect;
   private boolean mDoneFlinging;
@@ -351,6 +352,8 @@ public void draw(Canvas canvas) {
         mEndBackground.draw(canvas);
       }
     }
+    getDrawingRect(mRect);
+    canvas.clipRect(mRect);
     super.draw(canvas);
   }