Plumb experiment flags through page_load_metrics and add a new observer

components/page_load_metrics/common/page_load_metrics_messages.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.
 
 // IPC messages for page load metrics.
 // Multiply-included message file, hence no include guard.
 
 #include "base/time/time.h"
 #include "components/page_load_metrics/common/page_load_timing.h"
 #include "ipc/ipc_message_macros.h"
 
 #define IPC_MESSAGE_START PageLoadMetricsMsgStart
 
 // See comments in page_load_timing.h for details on each field.
 IPC_STRUCT_TRAITS_BEGIN(page_load_metrics::PageLoadTiming)
   IPC_STRUCT_TRAITS_MEMBER(navigation_start)
   IPC_STRUCT_TRAITS_MEMBER(response_start)
   IPC_STRUCT_TRAITS_MEMBER(dom_loading)
   IPC_STRUCT_TRAITS_MEMBER(dom_content_loaded_event_start)
   IPC_STRUCT_TRAITS_MEMBER(load_event_start)
   IPC_STRUCT_TRAITS_MEMBER(first_layout)
   IPC_STRUCT_TRAITS_MEMBER(first_paint)
   IPC_STRUCT_TRAITS_MEMBER(first_text_paint)
   IPC_STRUCT_TRAITS_MEMBER(first_image_paint)
   IPC_STRUCT_TRAITS_MEMBER(first_contentful_paint)
 IPC_STRUCT_TRAITS_END()
 
+IPC_STRUCT_TRAITS_BEGIN(page_load_metrics::PageLoadMetadata)
+  IPC_STRUCT_TRAITS_MEMBER(experiment_flags)
+IPC_STRUCT_TRAITS_END()
+
 // Sent from renderer to browser process when the PageLoadTiming for the
 // associated frame changed.
-IPC_MESSAGE_ROUTED1(PageLoadMetricsMsg_TimingUpdated,
-                    page_load_metrics::PageLoadTiming)
+IPC_MESSAGE_ROUTED2(PageLoadMetricsMsg_TimingUpdated,
+                    page_load_metrics::PageLoadTiming,
+                    page_load_metrics::PageLoadMetadata);