Add histogram to track overhead for scanning external css

third_party/WebKit/Source/core/html/parser/CSSPreloadScanner.cpp

 /*
  * Copyright (C) 2008, 2010 Apple Inc. All Rights Reserved.
  * Copyright (C) 2009 Torch Mobile, Inc. http://www.torchmobile.com/
  * Copyright (C) 2010 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.
@@ skipping 255 matching lines @@
 }
 
 void CSSPreloaderResourceClient::scanCSS(const CSSStyleSheetResource* resource)
 {
     DCHECK(m_preloader);
     // Passing an empty SegmentedString here results in PreloadRequest with no
     // file/line information.
     // TODO(csharrison): If this becomes an issue the CSSPreloadScanner may be
     // augmented to take care of this case without performing an additional
     // copy.
+    double startTime = monotonicallyIncreasingTimeMS();
     const String& chunk = resource->decodedText();
     if (chunk.isNull())
         return;
     CSSPreloadScanner cssPreloadScanner;
     PreloadRequestStream preloads;
     cssPreloadScanner.scan(chunk, SegmentedString(), preloads, resource->response().url());
+    DEFINE_STATIC_LOCAL(CustomCountHistogram, cssScanTimeHistogram, ("PreloadScanner.ExternalCSS.ScanTime", 1, 1000000, 50));
+    cssScanTimeHistogram.count((monotonicallyIncreasingTimeMS() - startTime) * 1000);
     fetchPreloads(preloads);
 }
 
 void CSSPreloaderResourceClient::fetchPreloads(PreloadRequestStream& preloads)
 {
     if (preloads.size()) {
         m_preloader->document()->loader()->didObserveLoadingBehavior(WebLoadingBehaviorFlag::WebLoadingBehaviorCSSPreloadFound);
     }
 
     Settings* settings = m_preloader->document()->settings();
     DCHECK(settings);
     if (settings->cssExternalScannerPreload()) {
         int currentPreloadCount = m_preloader->countPreloads();
         m_preloader->takeAndPreload(preloads);
         DEFINE_STATIC_LOCAL(CustomCountHistogram, cssImportHistogram, ("PreloadScanner.ExternalCSS.PreloadCount", 1, 100, 50));
         cssImportHistogram.count(m_preloader->countPreloads() - currentPreloadCount);
     }
 }
 
 DEFINE_TRACE(CSSPreloaderResourceClient)
 {
     visitor->trace(m_preloader);
     ResourceOwner<CSSStyleSheetResource>::trace(visitor);
 }
 
 } // namespace blink