Add fine grained dropped video frames counters on sending side

4 new counters added to SendStatisticsProxy and reported to UMA and logs.

Bug: webrtc:8355
Change-Id: Idf9b8dfc295c92821e058a97cb3894dc6a446082
Reviewed-on: https://webrtc-review.googlesource.com/12260
Commit-Queue: Ilya Nikolaevskiy <ilnik@webrtc.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20347}

api/video_codecs/video_encoder.h

@@ -56,13 +56,25 @@ class EncodedImageCallback {
     bool drop_next_frame = false;
   };
 
+  // Used to signal the encoder about reason a frame is dropped.
+  // kDroppedByMediaOptimizations - dropped by MediaOptimizations (for rate
+  // limiting purposes).
+  // kDroppedByEncoder - dropped by encoder's internal rate limiter.
+  enum class DropReason : uint8_t {
+    kDroppedByMediaOptimizations,
+    kDroppedByEncoder
+  };
+
   // Callback function which is called when an image has been encoded.
   virtual Result OnEncodedImage(
       const EncodedImage& encoded_image,
       const CodecSpecificInfo* codec_specific_info,
       const RTPFragmentationHeader* fragmentation) = 0;
 
+  // Deprecated. TODO(ilnik): Remove this in few weeks.
   virtual void OnDroppedFrame() {}
+
+  virtual void OnDroppedFrame(DropReason reason) {}
 };
 
 class VideoEncoder {