Add AshDesktopMediaList and enable Desktop Capture API on Chrome OS.

ui/snapshot/snapshot_android.cc

Index: ui/snapshot/snapshot_android.cc
diff --git a/ui/snapshot/snapshot_android.cc b/ui/snapshot/snapshot_android.cc
index f175a87ce743cb9f04d0c2ae7dd0be91da0ecebd..8dd309ad1833b7ba087e2b43379f424e693768d5 100644
--- a/ui/snapshot/snapshot_android.cc
+++ b/ui/snapshot/snapshot_android.cc
@@ -4,6 +4,7 @@
 
 #include "ui/snapshot/snapshot.h"
 
+#include "base/callback.h"
 #include "ui/base/android/view_android.h"
 #include "ui/base/android/window_android.h"
 #include "ui/gfx/display.h"
@@ -32,4 +33,13 @@ bool GrabWindowSnapshot(gfx::NativeWindow window,
       scaled_bounds.height(), png_representation);
 }
 
+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