Add AshDesktopMediaList and enable Desktop Capture API on Chrome OS.

ui/snapshot/snapshot_gtk.cc

Index: ui/snapshot/snapshot_gtk.cc
diff --git a/ui/snapshot/snapshot_gtk.cc b/ui/snapshot/snapshot_gtk.cc
index 8a2f8c1edd7003be501dde06f4e26109e1c7c07a..4f3f83378bbff50ee44b2999acb4d010e0c618a2 100644
--- a/ui/snapshot/snapshot_gtk.cc
+++ b/ui/snapshot/snapshot_gtk.cc
@@ -7,6 +7,7 @@
 #include <gdk/gdkx.h>
 #include <gtk/gtk.h>
 
+#include "base/callback.h"
 #include "base/logging.h"
 #include "ui/base/x/x11_util.h"
 #include "ui/gfx/rect.h"
@@ -82,4 +83,13 @@ bool GrabWindowSnapshot(gfx::NativeWindow window_handle,
       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