Add a use counter for Link rel=serviceworker headers.

third_party/WebKit/Source/core/frame/UseCounter.h

 /*
  * Copyright (C) 2012 Google, Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 1373 matching lines @@
     GenericSensorStart = 1680,
     GenericSensorStop = 1681,
     TouchEventPreventedNoTouchAction = 1682,
     TouchEventPreventedForcedDocumentPassiveNoTouchAction = 1683,
     V8Event_StopPropagation_Method = 1684,
     V8Event_StopImmediatePropagation_Method = 1685,
     ImageCaptureConstructor = 1686,
     V8Document_RootScroller_AttributeGetter = 1687,
     V8Document_RootScroller_AttributeSetter = 1688,
     CustomElementRegistryDefine = 1689,
+    LinkHeaderServiceWorker = 1690,
 
     // Add new features immediately above this line. Don't change assigned
     // numbers of any item, and don't reuse removed slots.
     // Also, run update_use_counter_feature_enum.py in
     // chromium/src/tools/metrics/histograms/ to update the UMA mapping.
     NumberOfFeatures,  // This enum value must be last.
   };
 
   // "count" sets the bit for this feature to 1. Repeated calls are ignored.
   static void count(const Frame*, Feature);
@@ skipping 68 matching lines @@
     // Tracks what features/properties need to be reported to the legacy
     // histograms.
     BitVector m_featureBits;
     BitVector m_CSSBits;
   } m_legacyCounter;
 };
 
 }  // namespace blink
 
 #endif  // UseCounter_h