Plumb experiment flags through page_load_metrics and add a new observer

components/page_load_metrics/common/page_load_timing.cc

Index: components/page_load_metrics/common/page_load_timing.cc
diff --git a/components/page_load_metrics/common/page_load_timing.cc b/components/page_load_metrics/common/page_load_timing.cc
index a479f7032f70c9f79f89ab0e2fe1f55996daf927..346da4a3a145c61cb9b9995d6cc9db1f5468fc4f 100644
--- a/components/page_load_metrics/common/page_load_timing.cc
+++ b/components/page_load_metrics/common/page_load_timing.cc
@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 #include "components/page_load_metrics/common/page_load_timing.h"
+#include "third_party/WebKit/public/platform/WebExperimentData.h"
 
 namespace page_load_metrics {
 
@@ -33,4 +34,11 @@ bool PageLoadTiming::IsEmpty() const {
          first_image_paint.is_zero();
 }
 
+PageLoadMetadata::PageLoadMetadata()
+    : experiment_flags(blink::WebExperimentData::NoExperiment) {}
+
+bool PageLoadMetadata::operator==(const PageLoadMetadata& other) const {
+  return experiment_flags == other.experiment_flags;
+}
+
 }  // namespace page_load_metrics