Moving Obj-C++ code in desktop_capture_objc.

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).

NOTRY=True

Bug: webrtc:7743
Change-Id: I19f8bb8719cfc6af259819e2089cebea72b5d531
Reviewed-on: https://webrtc-review.googlesource.com/2220
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Henrik Kjellander <kjellander@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#19899}
diff --git a/modules/desktop_capture/BUILD.gn b/modules/desktop_capture/BUILD.gn
index d08e69d..a87cbc1 100644
--- a/modules/desktop_capture/BUILD.gn
+++ b/modules/desktop_capture/BUILD.gn
@@ -184,6 +184,8 @@
       "mouse_cursor_monitor_mac.mm",
       "screen_capturer_mac.mm",
       "window_capturer_mac.mm",
+      "window_finder_mac.h",
+      "window_finder_mac.mm",
     ]
     deps = [
       ":desktop_capture_generic",
@@ -292,8 +294,6 @@
     "win/window_capture_utils.h",
     "window_capturer_win.cc",
     "window_finder.h",
-    "window_finder_mac.h",
-    "window_finder_mac.mm",
     "window_finder_win.cc",
     "window_finder_win.h",
   ]