Add AshDesktopMediaList and enable Desktop Capture API on Chrome OS.

ui/snapshot/snapshot_ios.mm

Index: ui/snapshot/snapshot_ios.mm
diff --git a/ui/snapshot/snapshot_ios.mm b/ui/snapshot/snapshot_ios.mm
index b373a0367b538cde942bec6de06b133daf4b99d5..af8cb4772c63f2dba7cd18ba986a4b036b9e3a45 100644
--- a/ui/snapshot/snapshot_ios.mm
+++ b/ui/snapshot/snapshot_ios.mm
@@ -4,6 +4,7 @@
 
 #include "ui/snapshot/snapshot.h"
 
+#include "base/callback.h"
 #include "ui/gfx/rect.h"
 
 namespace ui {
@@ -22,4 +23,13 @@ bool GrabWindowSnapshot(gfx::NativeWindow window,
   return false;
 }
 
+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