Create new webrtc/sdk subdirectory.

This will be the staging around for languages other than C++. Right now
only ObjC is planned in the near future.

BUG=

Review URL: https://codereview.webrtc.org/1904343002

Cr-Commit-Position: refs/heads/master@{#12496}
diff --git a/webrtc/sdk/OWNERS b/webrtc/sdk/OWNERS
new file mode 100644
index 0000000..cd06158
--- /dev/null
+++ b/webrtc/sdk/OWNERS
@@ -0,0 +1 @@
+tkchin@webrtc.org