Make magjed@ owner of webrtc/sdk/objc

Also, move owner file from webrtc/sdk to webrtc/sdk/objc since we now
also have webrtc/sdk/android directory.

BUG=None
TBR=tkchin
NOTRY=True

Review-Url: https://codereview.webrtc.org/2589233002
Cr-Commit-Position: refs/heads/master@{#15961}

webrtc/sdk/objc/OWNERS

@@ -0,0 +1,7 @@
+magjed@webrtc.org
+tkchin@webrtc.org
+
+# These are for the common case of adding or renaming files. If you're doing
+# structural changes, please get a review from a reviewer in this file.
+per-file *.gn=*
+per-file *.gni=*