[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"
 
@@ skipping 53 matching lines @@
 
   // Returns the time elapsed since the previous start. This method may only
   // be called on a previously started timer.
   TimeDelta Elapsed() const {
     DCHECK(IsStarted());
     TimeDelta elapsed = Now() - start_ticks_;
     DCHECK(elapsed.InMicroseconds() >= 0);
     return elapsed;
   }
 
+  // Move the start_ticks_ to adjust the current timer.
+  void AdjustBy(TimeDelta delta) {
+    DCHECK(IsStarted());
+    start_ticks_ += delta;
+  }
+
   // 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;
   }
 
   TimeTicks start_ticks_;
 #ifdef DEBUG
   bool started_;
 #endif
 };
 
 }  // namespace base
 }  // namespace v8
 
 #endif  // V8_BASE_PLATFORM_ELAPSED_TIMER_H_