message: change ms to us and make timestamp uint64
  1. microsecond usually is denoted as us; ms is millisecond
  2. make timestamp uint64 all the way
diff: @@ -37,9 +37,9 @@ struct ProfileEvent {
  // Label of the event. This usually describes the event.
  const char* tag;
  // Timestamp in microseconds when the event began.
  - int64_t begin_timestamp_ms;
  + uint64_t begin_timestamp_us;
  // Timestamp in microseconds when the event ended.
  - int64_t end_timestamp_ms;
  + uint64_t end_timestamp_us;
  // The field containing the type of event. This must be one of the event types
  // in EventType.
  EventType event_type;
  @@ -74,13 +74,13 @@ class ProfileBuffer {
  if (!enabled_) {
  return kInvalidEventHandle;
  }
  - int64_t timestamp = NowMicros();
  + uint64_t timestamp = NowMicros();
  int index = current_index_ % event_buffer_.size();
  event_buffer_[index].tag = tag;
  event_buffer_[index].event_type = event_type;
  event_buffer_[index].event_metadata = event_metadata;
  - event_buffer_[index].begin_timestamp_ms = timestamp;
  - event_buffer_[index].end_timestamp_ms = 0;
  + event_buffer_[index].begin_timestamp_us = timestamp;
  + event_buffer_[index].end_timestamp_us = 0;
  current_index_++;
  return index;
  }
  @@ -103,7 +103,7 @@ class ProfileBuffer {
  }
  int event_index = event_handle % max_size;
  - event_buffer_[event_index].end_timestamp_ms = NowMicros();
  + event_buffer_[event_index].end_timestamp_us = NowMicros();
  }
  // Returns the size of the buffer.
  @@ -134,7 +134,7 @@ class ProfileBuffer {
  }
  private:
  - static int64_t NowMicros() {
  + static uint64_t NowMicros() {
  // TODO(shashishekhar): Refactor this to a separate file.
  struct timeval tv;
  gettimeofday(&tv, nullptr);
  