Centre text in case where the distance between baselines is larger th…

Summary:
…an the font size (per CSS spec)

Extending upon work facebook#7603
Closes facebook#8851

![screen shot 2016-08-04 at 14 49 21](https://cloud.githubusercontent.com/assets/7275322/17404165/bd67bc48-5a52-11e6-9ba1-5a8524f18867.png)
![screen shot 2016-08-04 at 14 49 24](https://cloud.githubusercontent.com/assets/7275322/17404167/be5f4044-5a52-11e6-9014-391349f9c5e1.png)
Closes facebook#9211

Differential Revision: D3706347

fbshipit-source-id: 0adfff8e8418b02f9b5d6671f5c89669e41abec3

Libraries/Text/RCTShadowText.m

@@ -339,7 +339,9 @@ - (NSAttributedString *)_attributedStringWithFontFamily:(NSString *)fontFamily
   [self _addAttribute:NSFontAttributeName withValue:font toAttributedString:attributedString];
   [self _addAttribute:NSKernAttributeName withValue:letterSpacing toAttributedString:attributedString];
   [self _addAttribute:RCTReactTagAttributeName withValue:self.reactTag toAttributedString:attributedString];
-  [self _setParagraphStyleOnAttributedString:attributedString heightOfTallestSubview:heightOfTallestSubview];
+  [self _setParagraphStyleOnAttributedString:attributedString
+                              fontLineHeight:font.lineHeight
+                      heightOfTallestSubview:heightOfTallestSubview];
 
   // create a non-mutable attributedString for use by the Text system which avoids copies down the line
   _cachedAttributedString = [[NSAttributedString alloc] initWithAttributedString:attributedString];
@@ -362,6 +364,7 @@ - (void)_addAttribute:(NSString *)attribute withValue:(id)attributeValue toAttri
  * varying lineHeights, we simply take the max.
  */
 - (void)_setParagraphStyleOnAttributedString:(NSMutableAttributedString *)attributedString
+                                    fontLineHeight:(CGFloat)fontLineHeight
                       heightOfTallestSubview:(CGFloat)heightOfTallestSubview
 {
   // check if we have lineHeight set on self
@@ -431,6 +434,12 @@ - (void)_setParagraphStyleOnAttributedString:(NSMutableAttributedString *)attrib
     [attributedString addAttribute:NSParagraphStyleAttributeName
                              value:paragraphStyle
                              range:(NSRange){0, attributedString.length}];
+
+    if (lineHeight > fontLineHeight) {
+      [attributedString addAttribute:NSBaselineOffsetAttributeName
+                               value:@(lineHeight / 2 - fontLineHeight / 2)
+                               range:(NSRange){0, attributedString.length}];
+    }
   }
 
   // Text decoration