Move ScreenCapturerMac to mac sub directory

1- git mv screen_capturer_mac.mm  mac/screen_capturer_mac.mm
2- extract class ScreenCapturerMac declaritions to its own header
3- extract static CreateRawScreenCapturer to screen_capturer_darwin.mm
   (Using 'darwin' instead of 'mac' allows to make happy the command
   git log --follow mac/screen_capturer_mac.mm)
4- git cl format

Bug: webrtc:8652
Change-Id: Ibb13bd5dec61aa9b92c9f5f30fedd0508a727dd9
Reviewed-on: https://webrtc-review.googlesource.com/64680
Commit-Queue: Julien Isorce <julien.isorce@chromium.org>
Reviewed-by: Zijie He <zijiehe@chromium.org>
Cr-Commit-Position: refs/heads/master@{#22620}
diff --git a/modules/desktop_capture/BUILD.gn b/modules/desktop_capture/BUILD.gn
index bd0834c..6a10b56 100644
--- a/modules/desktop_capture/BUILD.gn
+++ b/modules/desktop_capture/BUILD.gn
@@ -175,8 +175,10 @@
     visibility = [ ":desktop_capture" ]
     sources = [
       "mac/desktop_configuration.mm",
+      "mac/screen_capturer_mac.h",
+      "mac/screen_capturer_mac.mm",
       "mouse_cursor_monitor_mac.mm",
-      "screen_capturer_mac.mm",
+      "screen_capturer_darwin.mm",
       "window_capturer_mac.mm",
       "window_finder_mac.h",
       "window_finder_mac.mm",