Move kMinPixelsPerFrame constant in VideoStreamEncoder to VideoEncoder::ScalingSettings.

Make it possible for forced VP8 SW fallback encoder to set min_pixels_per_frame via GetScalingSettings().

Add a min required resolution (in addition to bitrate) before releasing forced SW fallback.

BUG=webrtc:6634

Review-Url: https://codereview.webrtc.org/3000693003
Cr-Commit-Position: refs/heads/master@{#19390}

webrtc/modules/video_coding/codecs/vp8/vp8_impl.h

@@ -94,6 +94,7 @@ class VP8EncoderImpl : public VP8Encoder {
   uint32_t MaxIntraTarget(uint32_t optimal_buffer_size);
 
   const bool use_gf_boost_;
+  const rtc::Optional<int> min_pixels_per_frame_;
 
   EncodedImageCallback* encoded_complete_callback_;
   VideoCodec codec_;