[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..87dad2b663787d9f73d28037b6ca04cd1ac0965e 100644
--- a/src/base/platform/elapsed-timer.h
+++ b/src/base/platform/elapsed-timer.h
@@ -71,6 +71,12 @@ class ElapsedTimer final {
     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.