Make shelf item can be dragged when context menu is opened.

ui/views/controls/menu/menu_runner.h

Index: ui/views/controls/menu/menu_runner.h
diff --git a/ui/views/controls/menu/menu_runner.h b/ui/views/controls/menu/menu_runner.h
index abba98e9dd4095862b385ff57b26a6b49f3e2789..07c8c63321a38ed1703e07be108f8c4f8bd6259b 100644
--- a/ui/views/controls/menu/menu_runner.h
+++ b/ui/views/controls/menu/menu_runner.h
@@ -89,6 +89,12 @@ class VIEWS_EXPORT MenuRunner {
     // Menu with fixed anchor position, so |MenuRunner| will not attempt to
     // adjust the anchor point. For example the context menu of shelf item.
     FIXED_ANCHOR = 1 << 6,
+
+    // Owner of the menu could be in the middle of a gesture when the menu opens

[msw, 2017/06/01 20:24:28]

nit: "The menu's owner could"

[minch1, 2017/06/23 23:13:03]

Done.

+    // and can use this flag to continue the gesture. For example, shelf uses

[msw, 2017/06/01 20:24:28]

nit: "Chrome OS's shelf"

[minch1, 2017/06/23 23:13:03]

Done.

+    // the flag to continue dragging an item without lifting the finger after
+    // the context menu of the item is opened.
+    PRESERVE_CURRENT_GESTURE_FOR_OWNER = 1 << 7,

[msw, 2017/06/01 20:24:28]

optional nit: keep name in sync with MenuController's member (if renamed)

[minch1, 2017/06/23 23:13:03]

Done.

   };
 
   // Creates a new MenuRunner, which may use a native menu if available.