Use initial bitrates for software VP8.

Makes the software encoder start at VGA as well, since ~300k isn't good
enough to produce a good HD stream.

BUG=webrtc:5678
R=glaznev@webrtc.org, stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1893313002 .

Cr-Commit-Position: refs/heads/master@{#12428}

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

@@ -606,9 +606,8 @@ int VP8EncoderImpl::InitEncode(const VideoCodec* inst,
   // go back down).
   const int kLowQpThreshold = 29;
   const int kDisabledBadQpThreshold = 100;
-  // TODO(glaznev/sprang): consider passing codec initial bitrate to quality
-  // scaler to avoid starting with HD for low initial bitrates.
-  quality_scaler_.Init(kLowQpThreshold, kDisabledBadQpThreshold, false, 0, 0, 0,
+  quality_scaler_.Init(kLowQpThreshold, kDisabledBadQpThreshold, false,
+                       codec_.startBitrate, codec_.width, codec_.height,
                        codec_.maxFramerate);
 
   // Only apply scaling to improve for single-layer streams. The scaling metrics