Measure performance for ASCellNode layout (#2209)

* Measure performance for ASCellNode layout

* Address Adlai and Levi's feedback:
* Move to ASDisplayNode level
* Lock around setting/getting measurement options and results
* Record all measurement passes and report times as an array
* Only add relevant entries to the performanceMetrics dictionary
* Rebase

* Store sum and count instead of array

* Rename ScopeTimerDataPoint to ScopeTimerSum

* Address Levi's feedback

* Address Adlai's feedback

AsyncDisplayKit/Private/ASDisplayNodeInternal.h

@@ -22,6 +22,8 @@
 #import "ASEnvironment.h"
 #import "ASObjectDescriptionHelpers.h"
 
+#import "ASDisplayNode+Beta.h"
+
 @protocol _ASDisplayLayerDelegate;
 @class _ASDisplayLayer;
 @class _ASPendingState;
@@ -163,6 +165,13 @@ FOUNDATION_EXPORT NSString * const ASRenderingEngineDidDisplayNodesScheduledBefo
   CGPoint _accessibilityActivationPoint;
   UIBezierPath *_accessibilityPath;
 
+  // performance measurement
+  ASDisplayNodePerformanceMeasurementOptions _measurementOptions;
+  NSTimeInterval _layoutSpecTotalTime;
+  NSUInteger _layoutSpecNumberOfPasses;
+  NSTimeInterval _layoutGenerationTotalTime;
+  NSUInteger _layoutGenerationNumberOfPasses;
+
 #if TIME_DISPLAYNODE_OPS
 @public
   NSTimeInterval _debugTimeToCreateView;