Handle UNPIN operation in ash.

ash/accelerators/accelerator_commands.cc

Index: ash/accelerators/accelerator_commands.cc
diff --git a/ash/accelerators/accelerator_commands.cc b/ash/accelerators/accelerator_commands.cc
index 94cc0566860e41e0fe8d14d668d3347c1ff8f4f8..5ab32bf924ca8566fc403036c6e003dfbe45e636 100644
--- a/ash/accelerators/accelerator_commands.cc
+++ b/ash/accelerators/accelerator_commands.cc
@@ -12,6 +12,7 @@
 #include "ash/display/display_util.h"
 #include "ash/shell.h"
 #include "ash/shell_delegate.h"
+#include "ash/wm/screen_pinning_controller.h"
 #include "ash/wm/window_state_aura.h"
 #include "ash/wm/window_util.h"
 #include "base/metrics/user_metrics.h"
@@ -107,5 +108,12 @@ void ResetInternalDisplayZoom() {
   }
 }
 
+void Unpin() {
+  WmWindow* pinned_window =
+      Shell::GetInstance()->screen_pinning_controller()->pinned_window();
+  if (pinned_window)
+    pinned_window->GetWindowState()->Restore();
+}
+
 }  // namespace accelerators
 }  // namespace ash