Add NavigationToFirstContentfulPaint UMA for ServiceWorker controlled pages.

third_party/WebKit/Source/web/WebPerformance.cpp

Index: third_party/WebKit/Source/web/WebPerformance.cpp
diff --git a/third_party/WebKit/Source/web/WebPerformance.cpp b/third_party/WebKit/Source/web/WebPerformance.cpp
index baeb02c2f1b8d43c9dadf499609be88184a5c3bb..b34aeb0978482223213b15fa027491845c6de6b3 100644
--- a/third_party/WebKit/Source/web/WebPerformance.cpp
+++ b/third_party/WebKit/Source/web/WebPerformance.cpp
@@ -75,6 +75,11 @@ double WebPerformance::unloadEventEnd() const
     return millisecondsToSeconds(m_private->timing()->unloadEventEnd());
 }
 
+double WebPerformance::workerStart() const
+{
+    return millisecondsToSeconds(m_private->timing()->workerStart());
+}
+
 double WebPerformance::redirectStart() const
 {
     return millisecondsToSeconds(m_private->timing()->redirectStart());