Add UMA to track time between preload discovery and first bytes of response

third_party/WebKit/Source/core/fetch/Resource.cpp

Index: third_party/WebKit/Source/core/fetch/Resource.cpp
diff --git a/third_party/WebKit/Source/core/fetch/Resource.cpp b/third_party/WebKit/Source/core/fetch/Resource.cpp
index 653d09c4caee83e947d5a18733f283eaf1f428ce..568f7c98de2a0d9436acf2b5466b159a1eb15161 100644
--- a/third_party/WebKit/Source/core/fetch/Resource.cpp
+++ b/third_party/WebKit/Source/core/fetch/Resource.cpp
@@ -582,6 +582,11 @@ bool Resource::unlock()
 void Resource::responseReceived(const ResourceResponse& response, PassOwnPtr<WebDataConsumerHandle>)
 {
     m_responseTimestamp = currentTime();
+    if (m_preloadDiscoveryTime) {
+        int timeSinceDiscovery = static_cast<int>(1000 * (monotonicallyIncreasingTime() - m_preloadDiscoveryTime));
+        DEFINE_STATIC_LOCAL(CustomCountHistogram, preloadDiscoveryToFirstByteHistogram, ("PreloadScanner.TTFB", 0, 10000, 50));
+        preloadDiscoveryToFirstByteHistogram.count(timeSinceDiscovery);
+    }
 
     if (!m_revalidatingRequest.isNull()) {
         if (response.httpStatusCode() == 304) {