Move class VideoCodec from common_types.h to its own api header file.

Bug: webrtc:7660
Change-Id: I91f19bfc2565461328f30081f8383e136419aefb
Reviewed-on: https://webrtc-review.googlesource.com/79881
Commit-Queue: Niels Moller <nisse@webrtc.org>
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23544}

BUILD.gn

@@ -383,9 +383,8 @@ rtc_source_set("typedefs") {
   ]
 }
 
-rtc_static_library("webrtc_common") {
+rtc_source_set("webrtc_common") {
   sources = [
-    "common_types.cc",
     "common_types.h",
   ]
   deps = [