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

This is a reland of efc71e565e9b36bcdfb4571f59e34bbd8fabd0cd

Differs from the original cl by not widening the type of
VideoCodec::width and VideoCodec::height.

Original change's description:
> 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}

Bug: webrtc:7660
Change-Id: I7cf74a85a61ea2b831e6f32b3b3e17514ebefec8
Reviewed-on: https://webrtc-review.googlesource.com/82140
Commit-Queue: Niels Moller <nisse@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23569}

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 = [