Move RTCAVFoundationCapturer to webrtc/api/objc. BUG= R=tkchin@webrtc.org Review URL: https://codereview.webrtc.org/1559753002 . Patch from Jon Hjelle <hjon@andyet.net>. Cr-Original-Commit-Position: refs/heads/master@{#11348} Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc Cr-Mirrored-Commit: 7ac8babbc6f2ed4b0395ecaaf153ec91d71560d5