Migrated RTPchange Gyp target to GN.
Migrated RTPchange Gyp target to GN.

BUG=6162

Review-Url: https://codereview.webrtc.org/2194213002
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#13601}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 76cbe19f8ac292c3e8fe6f217d63c60b534d9821

Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk