Add UMA metric of time to start the first render process

chrome/browser/metrics/first_web_contents_profiler.cc

Index: chrome/browser/metrics/first_web_contents_profiler.cc
diff --git a/chrome/browser/metrics/first_web_contents_profiler.cc b/chrome/browser/metrics/first_web_contents_profiler.cc
index e67831b79e6fce602d4809f1f063e0ebf8c325ad..dc87c1f8bcb447f36a317658a9dbe6a373ba1662 100644
--- a/chrome/browser/metrics/first_web_contents_profiler.cc
+++ b/chrome/browser/metrics/first_web_contents_profiler.cc
@@ -19,6 +19,8 @@
 #include "components/metrics/proto/profiler_event.pb.h"
 #include "components/startup_metric_utils/browser/startup_metric_utils.h"
 #include "content/public/browser/navigation_handle.h"
+#include "content/public/browser/render_frame_host.h"
+#include "content/public/browser/render_process_host.h"
 #include "content/public/browser/web_contents.h"
 #include "content/public/browser/web_contents_observer.h"
 #include "content/public/common/browser_side_navigation_policy.h"
@@ -106,7 +108,10 @@ void FirstWebContentsProfiler::DidFirstVisuallyNonEmptyPaint() {
 
   collected_paint_metric_ = true;
   startup_metric_utils::RecordFirstWebContentsNonEmptyPaint(
-      base::TimeTicks::Now());
+      base::TimeTicks::Now(), web_contents()
+                                  ->GetMainFrame()
+                                  ->GetProcess()
+                                  ->GetInitTimeForNavigationMetrics());

[gab, 2017/06/01 14:56:45]

Does the browser get this signal before? startup_metric_utils works by getting
individual signals (e.g. process start, message loop start, etc.) and then
computing metrics from them.

Would be preferable to keep it that way instead of passing two values into
RecordNonEmptyPaint() -- e.g. if we wanted RenderedStart->MainNavigationStart we
could not with this approach.

 
   metrics::TrackingSynchronizer::OnProfilingPhaseCompleted(
       metrics::ProfilerEventProto::EVENT_FIRST_NONEMPTY_PAINT);