[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..a861fe016175266d282bce4becc15255bedf34fd 100644
--- a/src/base/platform/elapsed-timer.h
+++ b/src/base/platform/elapsed-timer.h
@@ -71,6 +71,14 @@ class ElapsedTimer final {
     return elapsed;
   }
 
+  // Move the start_ticks_ to adjust the current timer.
+  void AdjustBy(TimeDelta delta) {
+    DCHECK(IsStarted());
+    // If the delta is negative we want to make the elapsed time smaller, hence
+    // we have to move the start_ticks_ in the opposite direction.
+    start_ticks_ -= delta;

[Igor Sheludko, 2016/11/18 09:55:45]

Maybe call it Subtract(delta) expecting non-negative values to make it more
clear and say that we are subtracting delta by adding to the start_ticks.
Otherwise there are too many negations involved :)

[Camillo Bruni, 2016/11/18 16:36:48]

What's not wrong with double double negation negations?? 

done.

+  }
+
   // 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.