ObjC: Rename VideoToolbox/decoder.cc to VideoToolbox/decoder.mm

This decoder is only used for iOS/Mac and it will simplify to make it
ObjC++ instead of C++, similar to how the encoder is .mm already.

Bug: None
Change-Id: I13f62f018432e9c23e7277eea29258a73e1590e1
Reviewed-on: https://chromium-review.googlesource.com/529084
Reviewed-by: Kári Tristan Helgason <kthelgason@webrtc.org>
Reviewed-by: Anders Carlsson <andersc@webrtc.org>
Commit-Queue: Magnus Jedvert <magjed@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#18513}

webrtc/sdk/BUILD.gn

@@ -540,8 +540,8 @@ if (is_ios || is_mac) {
 
     rtc_static_library("objc_videotoolbox") {
       sources = [
-        "objc/Framework/Classes/VideoToolbox/decoder.cc",
         "objc/Framework/Classes/VideoToolbox/decoder.h",
+        "objc/Framework/Classes/VideoToolbox/decoder.mm",
         "objc/Framework/Classes/VideoToolbox/encoder.h",
         "objc/Framework/Classes/VideoToolbox/encoder.mm",
         "objc/Framework/Classes/VideoToolbox/nalu_rewriter.cc",