Auto merge of #17046 - dbaron:bug1367577, r=Manishearth

Fix stylo bindings for Gecko changes to nsITheme constants in bug 1367577

These changes fix the stylo bindings for the changes to the Gecko nsITheme constants in [bug 1367577](https://bugzilla.mozilla.org/show_bug.cgi?id=1367577).

I believe there isn't anything in these changes that should affect the stylo build (as opposed to the servo build, which I've tested on try in that bug).

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components/style/properties/longhand/box.mako.rs

@@ -2340,7 +2340,7 @@ ${helpers.single_keyword("-moz-appearance",
                             -moz-window-titlebar-maximized
                          """,
                          gecko_ffi_name="mAppearance",
-                         gecko_constant_prefix="NS_THEME",
+                         gecko_constant_prefix="ThemeWidgetType_NS_THEME",
                          products="gecko",
                          spec="Nonstandard (https://developer.mozilla.org/en-US/docs/Web/CSS/-moz-appearance)",
                          animation_value_type="none")}