Plumb experiment flags through page_load_metrics and add a new observer

components/page_load_metrics/renderer/page_timing_metrics_sender.h

Index: components/page_load_metrics/renderer/page_timing_metrics_sender.h
diff --git a/components/page_load_metrics/renderer/page_timing_metrics_sender.h b/components/page_load_metrics/renderer/page_timing_metrics_sender.h
index 0f5e1c9de7146ef1c27b49710a3e8944a6db04b2..624345879484b9fb5a61878e2ec4ef6cc5530e92 100644
--- a/components/page_load_metrics/renderer/page_timing_metrics_sender.h
+++ b/components/page_load_metrics/renderer/page_timing_metrics_sender.h
@@ -8,6 +8,7 @@
 #include "base/macros.h"
 #include "base/memory/scoped_ptr.h"
 #include "components/page_load_metrics/common/page_load_timing.h"
+#include "third_party/WebKit/public/platform/WebExperimentData.h"
 
 namespace base {
 class Timer;
@@ -29,6 +30,7 @@ class PageTimingMetricsSender {
                           scoped_ptr<base::Timer> timer);
   ~PageTimingMetricsSender();
 
+  void DidUseExperiment(blink::WebExperimentData data);
   void Send(const PageLoadTiming& timing);
 
  protected:
@@ -42,6 +44,10 @@ class PageTimingMetricsSender {
   scoped_ptr<base::Timer> timer_;
   PageLoadTiming last_timing_;
 
+  // The the sender keep track of metadata as it comes in, because the sender is
+  // scoped to a single committed load.
+  PageLoadMetadata metadata_;
+
   DISALLOW_COPY_AND_ASSIGN(PageTimingMetricsSender);
 };