Reland "Add new latency_info for tracking browser composite time."

cc/base/rolling_time_delta_history.cc

Index: cc/base/rolling_time_delta_history.cc
diff --git a/cc/base/rolling_time_delta_history.cc b/cc/base/rolling_time_delta_history.cc
index db04f586b0bee590a2dbec07fbd44957dbd3a7fd..0f95cc5fe5270fec4ffc41b3b3584b6b9cd781d3 100644
--- a/cc/base/rolling_time_delta_history.cc
+++ b/cc/base/rolling_time_delta_history.cc
@@ -26,6 +26,10 @@ void RollingTimeDeltaHistory::InsertSample(base::TimeDelta time) {
   chronological_sample_deque_.push_back(it);
 }
 
+size_t RollingTimeDeltaHistory::SampleCount() {
+  return sample_set_.size();
+}
+
 void RollingTimeDeltaHistory::Clear() {
   chronological_sample_deque_.clear();
   sample_set_.clear();