[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..2a235e1282c8d07c6239155dfbfd4a26d451a312 100644
--- a/src/base/platform/elapsed-timer.h
+++ b/src/base/platform/elapsed-timer.h
@@ -52,9 +52,10 @@ 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 ticks) {

[Igor Sheludko, 2016/11/18 18:56:27]

s/ticks/now/ to be clear.

[Camillo Bruni, 2016/11/21 10:50:03]

done.

     DCHECK(IsStarted());
-    TimeTicks ticks = Now();
     TimeDelta elapsed = ticks - start_ticks_;
     DCHECK(elapsed.InMicroseconds() >= 0);
     start_ticks_ = ticks;
@@ -71,6 +72,14 @@ class ElapsedTimer final {
     return elapsed;
   }
 
+  // Move the start_ticks_ to adjust the current timer.
+  void Subtract(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;
+  }
+
   // 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.