BugFix: Title should be from scene.route.title

Summary:
Its a minor Bug fix for rendering Title in NavigationHeader
Closes https://github.com/facebook/react-native/pull/8437

Differential Revision: D3507319

Pulled By: ericvicenti

fbshipit-source-id: ff4aa36be3c4991be11aabecbaad952c015a60e2

Libraries/NavigationExperimental/NavigationTypeDefinition.js

@@ -23,6 +23,7 @@ export type NavigationGestureDirection = 'horizontal' | 'vertical';
 
 export type NavigationRoute = {
   key: string,
+  title?: string
 };
 
 export type NavigationState = {