Show a warning when using sync xhr.

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 530 matching lines @@
         QuotedAnimationName = 571,
         QuotedKeyframesRule = 572,
         SrcsetDroppedCandidate = 573,
         WindowPostMessage = 574,
         WindowPostMessageWithLegacyTargetOriginArgument = 575,
         RenderRuby = 576,
         CanvasRenderingContext2DCompositeOperationDarker = 577,
         ScriptElementWithInvalidTypeHasSrc = 578,
         TimelineStart = 579,
         ElementBaseURIFromXMLBase = 580,
+        XMLHttpRequestSynchronousInNonWorkerOutsideBeforeUnload = 581,
 
         // 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 Document&, Feature);
@@ skipping 61 matching lines @@
 
     void updateMeasurements();
 
     OwnPtr<BitVector> m_countBits;
     BitVector m_CSSFeatureBits;
 };
 
 } // namespace blink
 
 #endif // UseCounter_h