Add DesktopCapturer GetSourceList SelectSource FocusOnSelectedSource functions

webrtc/modules/desktop_capture/BUILD.gn

Index: webrtc/modules/desktop_capture/BUILD.gn
diff --git a/webrtc/modules/desktop_capture/BUILD.gn b/webrtc/modules/desktop_capture/BUILD.gn
index 19bc2379c028fc859e5f55f54682d1c578153c37..8c59cdbb29e4daa3ed1f50c7e637c95b2e1ef8f4 100644
--- a/webrtc/modules/desktop_capture/BUILD.gn
+++ b/webrtc/modules/desktop_capture/BUILD.gn
@@ -41,6 +41,7 @@ rtc_static_library("desktop_capture") {
     "desktop_and_cursor_composer.h",
     "desktop_capture_options.cc",
     "desktop_capture_options.h",
+    "desktop_capturer.cc",
     "desktop_capturer.h",
     "desktop_frame_win.cc",
     "desktop_frame_win.h",
@@ -60,6 +61,7 @@ rtc_static_library("desktop_capture") {
     "mouse_cursor_monitor_mac.mm",
     "mouse_cursor_monitor_win.cc",
     "screen_capture_frame_queue.h",
+    "screen_capturer.cc",
     "screen_capturer.h",
     "screen_capturer_helper.cc",
     "screen_capturer_helper.h",
@@ -98,6 +100,7 @@ rtc_static_library("desktop_capture") {
     "win/screen_capturer_win_magnifier.h",
     "win/window_capture_utils.cc",
     "win/window_capture_utils.h",
+    "window_capturer.cc",
     "window_capturer.h",
     "window_capturer_mac.mm",
     "window_capturer_win.cc",