Add histogram to track overhead for scanning external css

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

Index: third_party/WebKit/Source/core/html/parser/CSSPreloadScanner.cpp
diff --git a/third_party/WebKit/Source/core/html/parser/CSSPreloadScanner.cpp b/third_party/WebKit/Source/core/html/parser/CSSPreloadScanner.cpp
index 65a946e34961c0d11d06b38d4e55854c607cdfbf..4c95d82e3354ee0f7daa1da53014ef26669072c1 100644
--- a/third_party/WebKit/Source/core/html/parser/CSSPreloadScanner.cpp
+++ b/third_party/WebKit/Source/core/html/parser/CSSPreloadScanner.cpp
@@ -273,12 +273,15 @@ void CSSPreloaderResourceClient::scanCSS(const CSSStyleSheetResource* resource)
     // 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);
 }