Export method to get current status bar height

Summary:
Addresses #2515 .
Closes https://github.com/facebook/react-native/pull/5039

Reviewed By: svcscm

Differential Revision: D2799224

Pulled By: nicklockwood

fb-gh-sync-id: 8f0a0f229a1588b1033b0121868c1f82e4dc6684

React/Modules/RCTStatusBarManager.m

@@ -89,6 +89,13 @@ RCT_EXPORT_MODULE()
   [self emitEvent:@"statusBarFrameWillChange" forNotification:notification];
 }
 
+RCT_EXPORT_METHOD(getHeight:(RCTResponseSenderBlock)callback)
+{
+  callback(@[@{
+    @"height": @([UIApplication sharedApplication].statusBarFrame.size.height),
+  }]);
+}
+
 RCT_EXPORT_METHOD(setStyle:(UIStatusBarStyle)statusBarStyle
                   animated:(BOOL)animated)
 {