Plumb experiment flags through page_load_metrics and add a new observer

components/page_load_metrics/common/page_load_timing.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef COMPONENTS_PAGE_LOAD_METRICS_COMMON_PAGE_LOAD_TIMING_H_
 #define COMPONENTS_PAGE_LOAD_METRICS_COMMON_PAGE_LOAD_TIMING_H_
 
 #include "base/time/time.h"
 
 namespace page_load_metrics {
@@ skipping 38 matching lines @@
   base::TimeDelta first_text_paint;
   // Time when the first image is painted.
   base::TimeDelta first_image_paint;
   // Time when the first contentful thing (image, text, etc.) is painted.
   base::TimeDelta first_contentful_paint;
 
   // If you add additional members, also be sure to update operator==,
   // 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_