Add histogram for percentage of sent frames that are limited in resolution due to quality:

- "WebRTC.Video.QualityLimitedResolutionInPercent"

and if a frame is downscaled, the average number of times the frame is downscaled:
- "WebRTC.Video.QualityLimitedResolutionDownscales"

BUG=

Review URL: https://codereview.webrtc.org/1325153009

Cr-Commit-Position: refs/heads/master@{#10319}

webrtc/modules/video_coding/utility/include/quality_scaler.h

@@ -37,6 +37,7 @@ class QualityScaler {
   Resolution GetScaledResolution() const;
   const VideoFrame& GetScaledFrame(const VideoFrame& frame);
   int GetTargetFramerate() const;
+  int downscale_shift() const { return downscale_shift_; }
 
  private:
   void AdjustScale(bool up);