Adding `scrollWithoutAnimationTo` method for ScrollViews

Summary:
Implementing the consensus approach from the comments on
this PR:
https://github.com/facebook/react-native/pull/486

We use a boolean flag in the Obj-C code to determine whether
to animate or not, and then provide two public JS functions
that call the Obj-C with or without the flag.
Closes https://github.com/facebook/react-native/pull/509
Github Author: Charlie Cheever <ccheever@gmail.com>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

Libraries/Components/ScrollView/ScrollView.js

@@ -202,6 +202,14 @@ var ScrollView = React.createClass({
     );
   },
 
+  scrollWithoutAnimationTo: function(destY?: number, destX?: number) {
+    RCTUIManager.scrollWithoutAnimationTo(
+      this.getNodeHandle(),
+      destX || 0,
+      destY || 0
+    );
+  },
+
   render: function() {
     var contentContainerStyle = [
       this.props.horizontal && styles.contentContainerHorizontal,

React/Modules/RCTUIManager.m

@@ -1051,7 +1051,21 @@ static void RCTMeasureLayout(RCTShadowView *view,
   [self addUIBlock:^(RCTUIManager *uiManager, RCTSparseArray *viewRegistry){
     UIView *view = viewRegistry[reactTag];
     if ([view conformsToProtocol:@protocol(RCTScrollableProtocol)]) {
-      [(id<RCTScrollableProtocol>)view scrollToOffset:CGPointMake([offsetX floatValue], [offsetY floatValue])];
+      [(id<RCTScrollableProtocol>)view scrollToOffset:CGPointMake([offsetX floatValue], [offsetY floatValue]) animated:YES];
+    } else {
+      RCTLogError(@"tried to scrollToOffset: on non-RCTScrollableProtocol view %@ with tag %@", view, reactTag);
+    }
+  }];
+}
+
+- (void)scrollWithoutAnimationToOffsetWithView:(NSNumber *)reactTag scrollToOffsetX:(NSNumber *)offsetX offsetY:(NSNumber *)offsetY
+{
+    RCT_EXPORT(scrollWithoutAnimationTo);
+
+    [self addUIBlock:^(RCTUIManager *uiManager, RCTSparseArray *viewRegistry){
+        UIView *view = viewRegistry[reactTag];
+        if ([view conformsToProtocol:@protocol(RCTScrollableProtocol)]) {
+            [(id<RCTScrollableProtocol>)view scrollToOffset:CGPointMake([offsetX floatValue], [offsetY floatValue]) animated:NO];
         } else {
             RCTLogError(@"tried to scrollToOffset: on non-RCTScrollableProtocol view %@ with tag %@", view, reactTag);
         }