media assets: fix .webm files not including video/audio bit rates in metadata.

Fix .webm files not including the `FFmpeg:VideoBitRate` and `FFmpeg:AudioBitRate`
fields in the media_metadata table. This was because the .webm format
doesn't include the video or audio bit rates in the metadata, and
ffprobe doesn't calculate them either, so we have to calculate them
ourselves by hand.

Fixup for 523d7afdd.

app/logical/media_file/image.rb

@@ -132,7 +132,7 @@ class MediaFile::Image < MediaFile
 
   def preview_frame
     if is_animated?
-      FFmpeg.new(file).smart_video_preview
+      FFmpeg.new(self).smart_video_preview
     else
       self
     end
@@ -172,7 +172,7 @@ class MediaFile::Image < MediaFile
   end
 
   def video
-    FFmpeg.new(file)
+    FFmpeg.new(self)
   end
 
   memoize :image, :video, :preview_frame, :dimensions, :error, :metadata, :is_corrupt?, :is_animated_gif?, :is_animated_png?

app/logical/media_file/video.rb

@@ -5,7 +5,7 @@
 #
 # @see https://github.com/streamio/streamio-ffmpeg
 class MediaFile::Video < MediaFile
-  delegate :duration, :frame_count, :frame_rate, :has_audio?, :is_corrupt?, :major_brand, :pix_fmt, :video_codec, :video_stream, :video_streams, :audio_codec, :audio_stream, :audio_streams, :error, to: :video
+  delegate :duration, :frame_count, :frame_rate, :has_audio?, :is_corrupt?, :major_brand, :pix_fmt, :video_codec, :video_bit_rate, :video_stream, :video_streams, :audio_codec, :audio_bit_rate, :audio_stream, :audio_streams, :error, to: :video
 
   def dimensions
     [video.width, video.height]
@@ -23,10 +23,11 @@ class MediaFile::Video < MediaFile
       "FFmpeg:FrameCount" => frame_count,
       "FFmpeg:VideoCodec" => video_codec,
       "FFmpeg:VideoProfile" => video_stream[:profile],
+      "FFmpeg:VideoBitRate" => video_bit_rate,
       "FFmpeg:AudioCodec" => audio_codec,
       "FFmpeg:AudioProfile" => audio_stream[:profile],
       "FFmpeg:AudioLayout" => audio_stream[:channel_layout],
-      "FFmpeg:AudioBitRate" => audio_stream[:bit_rate],
+      "FFmpeg:AudioBitRate" => audio_bit_rate,
     }.compact_blank)
   end
 
@@ -53,7 +54,7 @@ class MediaFile::Video < MediaFile
   private
 
   def video
-    FFmpeg.new(file)
+    FFmpeg.new(self)
   end
 
   def preview_frame