Make some functions const (#832)

include/benchmark/benchmark.h

@@ -574,7 +574,7 @@ class State {
   void SetComplexityN(int64_t complexity_n) { complexity_n_ = complexity_n; }
 
   BENCHMARK_ALWAYS_INLINE
-  int64_t complexity_length_n() { return complexity_n_; }
+  int64_t complexity_length_n() const { return complexity_n_; }
 
   // If this routine is called with items > 0, then an items/s
   // label is printed on the benchmark report line for the currently

src/thread_manager.h

@@ -11,7 +11,7 @@ namespace internal {
 
 class ThreadManager {
  public:
-  ThreadManager(int num_threads)
+  explicit ThreadManager(int num_threads)
       : alive_threads_(num_threads), start_stop_barrier_(num_threads) {}
 
   Mutex& GetBenchmarkMutex() const RETURN_CAPABILITY(benchmark_mutex_) {

src/thread_timer.h

@@ -43,19 +43,19 @@ class ThreadTimer {
   bool running() const { return running_; }
 
   // REQUIRES: timer is not running
-  double real_time_used() {
+  double real_time_used() const {
     CHECK(!running_);
     return real_time_used_;
   }
 
   // REQUIRES: timer is not running
-  double cpu_time_used() {
+  double cpu_time_used() const {
     CHECK(!running_);
     return cpu_time_used_;
   }
 
   // REQUIRES: timer is not running
-  double manual_time_used() {
+  double manual_time_used() const {
     CHECK(!running_);
     return manual_time_used_;
   }