Refactor QualityScaler and MovingAverage

The MovingAverage class was very specific to the QualityScaler. This
commit generalizes the MovingAverage class to be useful in other
situations as well, and adapts the QualityScaler to use the new
MovingAverage.

BUG=webrtc:6304

Review-Url: https://codereview.webrtc.org/2310853002
Cr-Commit-Position: refs/heads/master@{#14207}
diff --git a/webrtc/modules/video_coding/BUILD.gn b/webrtc/modules/video_coding/BUILD.gn
index b73b6e7..979355d 100644
--- a/webrtc/modules/video_coding/BUILD.gn
+++ b/webrtc/modules/video_coding/BUILD.gn
@@ -103,6 +103,7 @@
     "utility/h264_bitstream_parser.h",
     "utility/ivf_file_writer.cc",
     "utility/ivf_file_writer.h",
+    "utility/moving_average.cc",
     "utility/moving_average.h",
     "utility/qp_parser.cc",
     "utility/qp_parser.h",