[counters] RuntimeStats: fix wrong bookkeeping when dynamically changing counters.

src/base/platform/elapsed-timer.h

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_BASE_PLATFORM_ELAPSED_TIMER_H_
 #define V8_BASE_PLATFORM_ELAPSED_TIMER_H_
 
 #include "src/base/logging.h"
 #include "src/base/platform/time.h"
 
 namespace v8 {
 namespace base {
 
 class ElapsedTimer final {
  public:
 #ifdef DEBUG
   ElapsedTimer() : started_(false) {}
 #endif
 
   // Starts this timer. Once started a timer can be checked with
   // |Elapsed()| or |HasExpired()|, and may be restarted using |Restart()|.
   // This method must not be called on an already started timer.
-  void Start() {
+  void Start() { Start(Now()); }
+  void Start(TimeTicks now) {
     DCHECK(!IsStarted());
-    start_ticks_ = Now();
+    start_ticks_ = now;
 #ifdef DEBUG
     started_ = true;
 #endif
     DCHECK(IsStarted());
   }
 
   // Stops this timer. Must not be called on a timer that was not
   // started before.
   void Stop() {
     DCHECK(IsStarted());
     start_ticks_ = TimeTicks();
 #ifdef DEBUG
     started_ = false;
 #endif
     DCHECK(!IsStarted());
   }
 
   // Returns |true| if this timer was started previously.
   bool IsStarted() const {
     DCHECK(started_ || start_ticks_.IsNull());
     DCHECK(!started_ || !start_ticks_.IsNull());
     return !start_ticks_.IsNull();
   }
 
   // Restarts the timer and returns the time elapsed since the previous start.
   // This method is equivalent to obtaining the elapsed time with |Elapsed()|
   // and then starting the timer again, but does so in one single operation,
   // avoiding the need to obtain the clock value twice. It may only be called
   // on a previously started timer.
-  TimeDelta Restart() {
+  TimeDelta Restart() { return Restart(Now()); }
+
+  TimeDelta Restart(TimeTicks now) {
     DCHECK(IsStarted());
-    TimeTicks ticks = Now();
-    TimeDelta elapsed = ticks - start_ticks_;
+    TimeDelta elapsed = now - start_ticks_;
     DCHECK(elapsed.InMicroseconds() >= 0);
-    start_ticks_ = ticks;
+    start_ticks_ = now;
     DCHECK(IsStarted());
     return elapsed;
   }
 
   // Returns the time elapsed since the previous start. This method may only
   // be called on a previously started timer.
-  TimeDelta Elapsed() const {
+  TimeDelta Elapsed() const { return Elapsed(Now()); }
+  TimeDelta Elapsed(TimeTicks now) const {
     DCHECK(IsStarted());
-    TimeDelta elapsed = Now() - start_ticks_;
+    TimeDelta elapsed = now - start_ticks_;
     DCHECK(elapsed.InMicroseconds() >= 0);
     return elapsed;
   }
 
   // Returns |true| if the specified |time_delta| has elapsed since the
   // previous start, or |false| if not. This method may only be called on
   // a previously started timer.
   bool HasExpired(TimeDelta time_delta) const {
     DCHECK(IsStarted());
     return Elapsed() >= time_delta;
   }
 
- private:
   static V8_INLINE TimeTicks Now() {
     TimeTicks now = TimeTicks::HighResolutionNow();
     DCHECK(!now.IsNull());
     return now;
   }
 
+ private:
   TimeTicks start_ticks_;
 #ifdef DEBUG
   bool started_;
 #endif
 };
 
 }  // namespace base
 }  // namespace v8
 
 #endif  // V8_BASE_PLATFORM_ELAPSED_TIMER_H_