Restore some files for H.264. (#196)

These files were removed from GN during rebasing. Restore them for
missing symbols.

Author: jianjunz

modules/video_coding/BUILD.gn

@@ -500,6 +500,10 @@ rtc_library("webrtc_h264") {
   visibility = [ "*" ]
   sources = [
     "codecs/h264/h264.cc",
+    "codecs/h264/h264_color_space.cc",
+    "codecs/h264/h264_color_space.h",
+    "codecs/h264/h264_decoder_impl.cc",
+    "codecs/h264/h264_decoder_impl.h",
     "codecs/h264/h264_encoder_impl.cc",
     "codecs/h264/h264_encoder_impl.h",
     "codecs/h264/include/h264.h",