Move VideoDecoderSoftwareFallbackWrapper from webrtc/video_decoder.h to webrtc/media/engine/

webrtc/media/BUILD.gn

Index: webrtc/media/BUILD.gn
diff --git a/webrtc/media/BUILD.gn b/webrtc/media/BUILD.gn
index 590450bd296f0ab0ff97ab4e63e86b9a08ea423d..8ed07b14a5572e8562f3ee0b10c4e1dfd5d55fa2 100644
--- a/webrtc/media/BUILD.gn
+++ b/webrtc/media/BUILD.gn
@@ -88,6 +88,8 @@ rtc_static_library("rtc_media") {
     "engine/payload_type_mapper.h",
     "engine/simulcast.cc",
     "engine/simulcast.h",
+    "engine/videodecodersoftwarefallbackwrapper.cc",
+    "engine/videodecodersoftwarefallbackwrapper.h",
     "engine/videoencodersoftwarefallbackwrapper.cc",
     "engine/videoencodersoftwarefallbackwrapper.h",
     "engine/webrtccommon.h",
@@ -294,6 +296,7 @@ if (rtc_include_tests) {
       "engine/nullwebrtcvideoengine_unittest.cc",
       "engine/payload_type_mapper_unittest.cc",
       "engine/simulcast_unittest.cc",
+      "engine/videodecodersoftwarefallbackwrapper_unittest.cc",
       "engine/videoencodersoftwarefallbackwrapper_unittest.cc",
       "engine/webrtcmediaengine_unittest.cc",
       "engine/webrtcvideocapturer_unittest.cc",