The goal of this CL is to separate Obj-C/Obj-C++ code from targets which have
also C++ code (see https://bugs.chromium.org/p/webrtc/issues/detail?id=7743
for more information).

To achieve this we have created 2 targets (desktop_capture_objc and
desktop_capture_generic) and desktop_capture will act as a proxy between these
targets (this way we can avoid a circular dependency between
desktop_capture_generic and desktop_capture_objc).

BUG=webrtc:7743

Review-Url: https://codereview.webrtc.org/2989053002
Cr-Original-Commit-Position: refs/heads/master@{#19607}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 13b9882fd3999d8394126017042d62bae719180b