Fixed ScrollView's .scrollToEnd to refrain from exceeding start bound…

…ary.

Summary:
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The problem occurs when a ScrollView's content height is smaller than the ScrollView height.  If the method `scrollToEnd` is called on the ScrollView, it will pull the content down until the bottom of the content is aligned with the bottom of the Scrollview container.

This fix will ensure the proper functionality: That the furthest the ScrollView can scroll down is to where the top of the content container is at the origin (i.e., the ScrollView scroll number cannot be less than 0).

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I tested on a scenario where the ScrollView is almost the full size of the screen, and the content of the ScrollView has a height of much less.  In this situation, the `scrollToEnd` method was executed and the content stayed in the same position.  This is the intended behavior.  If the content of the ScrollView is smaller than the height of the ScrollView, then the `scrollToEnd` method should not scroll anywhere.

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Closes facebook#12889

Reviewed By: javache

Differential Revision: D5289894

Pulled By: sahrens

fbshipit-source-id: df2e779ee855c1dea85d33649d754371ad244bca

React/Views/RCTScrollView.m

@@ -525,11 +525,11 @@ - (void)scrollToEnd:(BOOL)animated
   BOOL isHorizontal = [self isHorizontal:_scrollView];
   CGPoint offset;
   if (isHorizontal) {
-    CGFloat offsetX = _scrollView.contentSize.width - _scrollView.bounds.size.width;
-    offset = CGPointMake(MAX(offsetX, 0), 0);
+    CGFloat offsetX = _scrollView.contentSize.width - _scrollView.bounds.size.width + _scrollView.contentInset.right;
+    offset = CGPointMake(fmax(offsetX, 0), 0);
   } else {
-    CGFloat offsetY = _scrollView.contentSize.height - _scrollView.bounds.size.height;
-    offset = CGPointMake(0, MAX(offsetY, 0));
+    CGFloat offsetY = _scrollView.contentSize.height - _scrollView.bounds.size.height + _scrollView.contentInset.bottom;
+    offset = CGPointMake(0, fmax(offsetY, 0));
   }
   if (!CGPointEqualToPoint(_scrollView.contentOffset, offset)) {
     // Ensure at least one scroll event will fire