Add AshDesktopMediaList and enable Desktop Capture API on Chrome OS.

ui/snapshot/snapshot_mac.mm

Index: ui/snapshot/snapshot_mac.mm
diff --git a/ui/snapshot/snapshot_mac.mm b/ui/snapshot/snapshot_mac.mm
index abd87286a2d414158ef2472249864ca3216f8791..f86757bd1e493b4e87335199472210f0d8ffe4e4 100644
--- a/ui/snapshot/snapshot_mac.mm
+++ b/ui/snapshot/snapshot_mac.mm
@@ -6,6 +6,7 @@
 
 #import <Cocoa/Cocoa.h>
 
+#include "base/callback.h"
 #include "base/logging.h"
 #include "base/mac/scoped_cftyperef.h"
 #include "base/mac/scoped_nsobject.h"
@@ -72,4 +73,13 @@ bool GrabWindowSnapshot(gfx::NativeWindow window,
       snapshot_bounds);
 }
 
+SNAPSHOT_EXPORT void GrapWindowSnapshotAsync(
+    gfx::NativeWindow window,
+    const gfx::Rect& snapshot_bounds,
+    const gfx::Size& target_size,
+    scoped_refptr<base::TaskRunner> background_task_runner,
+    GrapWindowSnapshotAsyncCallback callback) {
+  NOTIMPLEMENTED();
+}
+
 }  // namespace ui