Set contentView's frame in setter

Summary:
In D20836890 we no longer set `_contentView.frame` inside `layoutSubviews`.

This doesn't work well with component views that set their `contentView` after `updateLayoutMetrics` is called. For Example legacy interop which sets its contentView in the `finalizeUpdates` method. Other component views that set their contentView during initialisation are fine.

Changelog: [Internal]

Reviewed By: shergin

Differential Revision: D21302108

fbshipit-source-id: 5adcf489e4e650ac04b84c836a8f8d6f55a77267

React/Fabric/Mounting/ComponentViews/View/RCTViewComponentView.mm

@@ -48,6 +48,7 @@ - (void)setContentView:(UIView *)contentView
 
   if (_contentView) {
     [self addSubview:_contentView];
+    _contentView.frame = RCTCGRectFromRect(_layoutMetrics.getContentFrame());
   }
 }