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

src/base/platform/elapsed-timer.h

Index: src/base/platform/elapsed-timer.h
diff --git a/src/base/platform/elapsed-timer.h b/src/base/platform/elapsed-timer.h
index f9a9ef43619c2650caf94bfdeefa71f969366db6..fc332d5a5a61fbfb2f3c4cf8eaf59c5d491d7f1a 100644
--- a/src/base/platform/elapsed-timer.h
+++ b/src/base/platform/elapsed-timer.h
@@ -20,9 +20,10 @@ class ElapsedTimer final {
   // 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
@@ -52,21 +53,23 @@ class ElapsedTimer final {
   // 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;
   }
@@ -79,13 +82,13 @@ class ElapsedTimer final {
     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_;