Add support for a forced software encoder fallback.

webrtc/media/engine/videoencodersoftwarefallbackwrapper.h

Index: webrtc/media/engine/videoencodersoftwarefallbackwrapper.h
diff --git a/webrtc/media/engine/videoencodersoftwarefallbackwrapper.h b/webrtc/media/engine/videoencodersoftwarefallbackwrapper.h
index 61c1a11c8d32e798b043c033c3cbecde17b91df8..ea7e13d353ba3846ae669b35e413f617bda8f78c 100644
--- a/webrtc/media/engine/videoencodersoftwarefallbackwrapper.h
+++ b/webrtc/media/engine/videoencodersoftwarefallbackwrapper.h
@@ -49,6 +49,33 @@ class VideoEncoderSoftwareFallbackWrapper : public VideoEncoder {
  private:
   bool InitFallbackEncoder();
 
+  // If |forced_fallback_possible_| is true:
+  // The forced fallback is requested if the target bitrate is below |low_kbps|
+  // for more than |min_low_ms| and the input video resolution is not larger
+  // than |kMaxPixels|.
+  // If the bitrate is above |high_kbps|, the forced fallback is requested to
+  // immediately be stopped.
+  class ForcedFallbackParams {
+   public:
+    bool ShouldStart(uint32_t bitrate_kbps, const VideoCodec& codec);
+    bool ShouldStop(uint32_t bitrate_kbps) const;
+    void Reset() { start_ms.reset(); }
+    bool IsValid(const VideoCodec& codec) const {
+      return codec.width * codec.height <= kMaxPixels;
+    }
+    rtc::Optional<int64_t> start_ms;  // Set when bitrate is below |low_kbps|.
+    uint32_t low_kbps = 100;
+    uint32_t high_kbps = 150;
+    int64_t min_low_ms = 10000;
+    const int kMaxPixels = 320 * 240;
+  };
+
+  bool RequestForcedFallback();
+  bool TryReleaseForcedFallbackEncoder();
+  bool TryReInitForcedFallbackEncoder();
+  void ValidateSettingsForForcedFallback();
+  bool IsForcedFallbackActive() const;
+
   // Settings used in the last InitEncode call and used if a dynamic fallback to
   // software is required.
   VideoCodec codec_settings_;
@@ -71,6 +98,9 @@ class VideoEncoderSoftwareFallbackWrapper : public VideoEncoder {
   std::unique_ptr<webrtc::VideoEncoder> fallback_encoder_;
   std::string fallback_implementation_name_;
   EncodedImageCallback* callback_;
+
+  bool forced_fallback_possible_;
+  ForcedFallbackParams forced_fallback_;
 };
 
 }  // namespace webrtc