NavigationTiming: set navigationStart for navigations in new tabs.

content/renderer/render_frame_impl.cc

Index: content/renderer/render_frame_impl.cc
diff --git a/content/renderer/render_frame_impl.cc b/content/renderer/render_frame_impl.cc
index 92e7367871040aa18c6a4c27ab77a3ff6f68fbff..dcc8c5ac53c80173cacd848ee649e12d9c80d48b 100644
--- a/content/renderer/render_frame_impl.cc
+++ b/content/renderer/render_frame_impl.cc
@@ -884,10 +884,12 @@ void RenderFrameImpl::OnNavigate(const FrameMsg_Navigate_Params& params) {
 
     frame->loadRequest(request);
 
-    // If this is a cross-process navigation, the browser process will send
-    // along the proper navigation start value.
-    if (!params.browser_navigation_start.is_null() &&
-        frame->provisionalDataSource()) {
+    // The browser provides the navigation_start time to bootstrap the
+    // Navigation Timing information for the browser-initiated navigations. In
+    // case of cross-process navigations, this carries over the time of
+    // finishing the onbeforeunload handler of the previous page.
+    DCHECK(!params.browser_navigation_start.is_null());
+    if (frame->provisionalDataSource()) {
       // browser_navigation_start is likely before this process existed, so we
       // can't use InterProcessTimeTicksConverter. Instead, the best we can do
       // is just ensure we don't report a bogus value in the future.