Add trace event for detecting main frame in loading metrics.

third_party/WebKit/Source/web/WebLocalFrameImpl.cpp

Index: third_party/WebKit/Source/web/WebLocalFrameImpl.cpp
diff --git a/third_party/WebKit/Source/web/WebLocalFrameImpl.cpp b/third_party/WebKit/Source/web/WebLocalFrameImpl.cpp
index 2a170a5d60cfe4692e83b3ab10392418383727d1..c8050a204e44d114facbb231b3ad6f62bedf7b51 100644
--- a/third_party/WebKit/Source/web/WebLocalFrameImpl.cpp
+++ b/third_party/WebKit/Source/web/WebLocalFrameImpl.cpp
@@ -1596,6 +1596,12 @@ void WebLocalFrameImpl::initializeCoreFrame(FrameHost* host,
 
     frame()->interfaceRegistry()->addInterface(WTF::bind(
         &InstallationServiceImpl::create, wrapWeakPersistent(frame())));
+    if (!owner) {
+      // This trace event is needed to detect the main frame of the
+      // renderer in telemetry metrics. See crbug.com/692112#c11.
+      TRACE_EVENT_INSTANT1("loading", "markAsMainFrame",
+                           TRACE_EVENT_SCOPE_THREAD, "frame", frame());
+    }
   }
 }