Fix content offset validation

Summary:
Content offset was broken because on initial render contentSize is {0,0} so any positive offset is lost. Also inset top/bottom and left/right were inversed �, this led to bad initial scrolling offset when using contentInset. This fixes it by making sure contentSize is actually measured (not {0,0}. I guess it's possible that the content is ACTUALLY {0,0} but in that case I don't think it really matters).

**Test plan**
Tested that a scrollview has proper scroll position when specifying contentOffset. Also tested that it works well with contentInset.
```js
<ScrollView contentOffset={{y: 100}}>
  <View style={{height: 1000}} />
</ScrollView>
```
Closes https://github.com/facebook/react-native/pull/15670

Differential Revision: D5771221

Pulled By: shergin

fbshipit-source-id: 455ed8fd5a4ad1ec61780b573d1a8ef1d77dd124

React/Views/RCTScrollView.m

@@ -315,12 +315,17 @@ RCT_NOT_IMPLEMENTED(- (instancetype)init)
 
   UIEdgeInsets contentInset = self.contentInset;
   CGSize contentSize = self.contentSize;
-
-  CGSize boundsSize = self.bounds.size;
-
-  self.contentOffset = CGPointMake(
-    MAX(-contentInset.top, MIN(contentSize.width - boundsSize.width + contentInset.bottom, originalOffset.x)),
-    MAX(-contentInset.left, MIN(contentSize.height - boundsSize.height + contentInset.right, originalOffset.y)));
+  
+  // If contentSize has not been measured yet we can't check bounds.
+  if (CGSizeEqualToSize(contentSize, CGSizeZero)) {
+    self.contentOffset = originalOffset;
+  } else {
+    // Make sure offset don't exceed bounds. This could happen on screen rotation.
+    CGSize boundsSize = self.bounds.size;
+    self.contentOffset = CGPointMake(
+      MAX(-contentInset.left, MIN(contentSize.width - boundsSize.width + contentInset.right, originalOffset.x)),
+      MAX(-contentInset.top, MIN(contentSize.height - boundsSize.height + contentInset.bottom, originalOffset.y)));
+  }
 }
 
 #if !TARGET_OS_TV