Log UMA for navigation start timestamp skew for browser-initiated loads

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 5a9da313bc67b3f7b2932be454a43784b50d4c8b..22687d65b135fe29602143f958169c2ce8f1be0b 100644
--- a/content/renderer/render_frame_impl.cc
+++ b/content/renderer/render_frame_impl.cc
@@ -473,9 +473,15 @@ base::TimeTicks SanitizeNavigationTiming(
   DCHECK(!browser_navigation_start.is_null());
   base::TimeTicks navigation_start =
       std::min(browser_navigation_start, renderer_navigation_start);
-  // TODO(csharrison) Investigate how big a problem the cross process
-  // monotonicity really is and on what platforms. Log UMA for:
-  // |renderer_navigation_start - browser_navigation_start|
+  base::TimeDelta difference =
+      renderer_navigation_start - browser_navigation_start;
+  if (difference > base::TimeDelta()) {
+    UMA_HISTOGRAM_TIMES("Navigation.Start.RendererBrowserDifference.Positive",
+                        difference);
+  } else {
+    UMA_HISTOGRAM_TIMES("Navigation.Start.RendererBrowserDifference.Negative",
+                        -difference);
+  }
   return navigation_start;
 }