NetEq: Move counting of generated CNG samples from DecisionLogic

The counting is moved to NetEqImpl, and the new counter is realized as a
Stopwatch object. The DecisionLogic class still has to maintain record
of when the CNG period is shortened, in order to reduce the delay. This
is recorded in a new noise_fast_forward_ member in DecisionLogic.

BUG=webrtc:5608

Review-Url: https://codereview.webrtc.org/1914303004
Cr-Commit-Position: refs/heads/master@{#12608}

webrtc/modules/audio_coding/neteq/neteq_impl.h

@@ -405,6 +405,8 @@ class NetEqImpl : public webrtc::NetEq {
   bool nack_enabled_ GUARDED_BY(crit_sect_);
   AudioFrame::VADActivity last_vad_activity_ GUARDED_BY(crit_sect_) =
       AudioFrame::kVadPassive;
+  std::unique_ptr<TickTimer::Stopwatch> generated_noise_stopwatch_
+      GUARDED_BY(crit_sect_);
 
  private:
   RTC_DISALLOW_COPY_AND_ASSIGN(NetEqImpl);