Add NavigationToFirstContentfulPaint UMA for ServiceWorker controlled pages.

third_party/WebKit/Source/core/timing/PerformanceTiming.h

Index: third_party/WebKit/Source/core/timing/PerformanceTiming.h
diff --git a/third_party/WebKit/Source/core/timing/PerformanceTiming.h b/third_party/WebKit/Source/core/timing/PerformanceTiming.h
index 2ebabc96d7bf309234513cf561993ad3ac15b515..5893c8a38e6af276074ebd0462be503886bfae02 100644
--- a/third_party/WebKit/Source/core/timing/PerformanceTiming.h
+++ b/third_party/WebKit/Source/core/timing/PerformanceTiming.h
@@ -79,6 +79,10 @@ public:
     unsigned long long loadEventStart() const;
     unsigned long long loadEventEnd() const;
 
+    // TODO(ksakamoto): This is a Navigation Timing Level 2's attribute and not
+    // exposed to JS currently. https://crbug.com/504237
+    unsigned long long workerStart() const;
+
     // The below are non-spec timings, for Page Load UMA metrics.
 
     // The time the first document layout is performed.