Plumb experiment flags through page_load_metrics and add a new observer

components/page_load_metrics/common/page_load_timing.h

Index: components/page_load_metrics/common/page_load_timing.h
diff --git a/components/page_load_metrics/common/page_load_timing.h b/components/page_load_metrics/common/page_load_timing.h
index 4336a2820484d903f77d89c5bc0a9125d437f93e..0ff5639a7c878c97929ff96a95cf6261e9d4cd8d 100644
--- a/components/page_load_metrics/common/page_load_timing.h
+++ b/components/page_load_metrics/common/page_load_timing.h
@@ -56,6 +56,13 @@ struct PageLoadTiming {
   // page_load_metrics_messages.h, and IsEmpty().
 };
 
+struct PageLoadMetadata {
+  PageLoadMetadata();
+  bool operator==(const PageLoadMetadata& other) const;
+  // These are packed blink::WebExperimentData enums.
+  int experiment_flags;

[Bryan McQuade, 2016/04/07 14:25:14]

nit: for structs with no constructor logic, i prefer setting defaults directly
here, rather than in the constructor. easier to reason about when everything is
in one place.

[Charlie Harrison, 2016/04/07 16:38:57]

Done.

+};
+
 }  // namespace page_load_metrics
 
 #endif  // COMPONENTS_PAGE_LOAD_METRICS_COMMON_PAGE_LOAD_TIMING_H_