NavigationTiming: set navigationStart for navigations in new tabs.

content/common/frame_messages.h

Index: content/common/frame_messages.h
diff --git a/content/common/frame_messages.h b/content/common/frame_messages.h
index d7664b7da34345e05bee13428a51e41d9134731e..ee3a9a29235a6c0e627290331142b0a175a71d73 100644
--- a/content/common/frame_messages.h
+++ b/content/common/frame_messages.h
@@ -209,7 +209,9 @@ IPC_STRUCT_BEGIN(FrameMsg_Navigate_Params)
   // Type of navigation.
   IPC_STRUCT_MEMBER(FrameMsg_Navigate_Type::Value, navigation_type)
 
-  // The time the request was created
+  // The time the request was created. This is used by the old performance
+  // infrastructure to set up DocumentState associated with the RenderView.
+  // TODO(ppi): make it go away.
   IPC_STRUCT_MEMBER(base::Time, request_time)
 
   // Extra headers (separated by \n) to send during the request.
@@ -242,7 +244,7 @@ IPC_STRUCT_BEGIN(FrameMsg_Navigate_Params)
   // If not empty, which frame to navigate.
   IPC_STRUCT_MEMBER(std::string, frame_to_navigate)
 
-  // The navigationStart time to expose to JS for this navigation.
+  // The navigationStart time to expose through the Navigation Timing API to JS.
   IPC_STRUCT_MEMBER(base::TimeTicks, browser_navigation_start)
 IPC_STRUCT_END()