Merge "libbacktrace: make Backtrace::FormatFrameData static." am: 48b68b49 am: 487f8890

  // Create a string representing the formatted line of backtrace information

  // for a single frame.

  virtual std::string FormatFrameData(size_t frame_num);

  virtual std::string FormatFrameData(const backtrace_frame_data_t* frame);

  static std::string FormatFrameData(const backtrace_frame_data_t* frame);

  pid_t Pid() const { return pid_; }

  pid_t Tid() const { return tid_; }