Bug 1479216: Restore the order of Scrollbarbutton appearance values. r=xidorn

The patch at bug 1478391 comment 6 changed the way the math in Scrollbarbutton*
worked, which pretty surely caused this.

Restore the original order and math to be the same as before bug 1478391.

MozReview-Commit-ID: CK3iOqeX2NW

layout/style/ServoStyleConsts.h

@@ -129,13 +129,14 @@ enum class StyleAppearance : uint8_t {
   // A scrollbar button (up/down/left/right).
   // Keep these in order (some code casts these values to `int` in order to
   // compare them against each other).
+  ScrollbarbuttonUp,
   ScrollbarbuttonDown,
   ScrollbarbuttonLeft,
   ScrollbarbuttonRight,
-  ScrollbarbuttonUp,
   // The scrollbar thumb.
   ScrollbarthumbHorizontal,
   ScrollbarthumbVertical,
+  // The scrollbar track.
   ScrollbartrackHorizontal,
   ScrollbartrackVertical,
   // The scroll corner

servo/components/style/values/specified/box.rs

@@ -999,10 +999,10 @@ pub enum Appearance {
     /// A scrollbar button (up/down/left/right).
     /// Keep these in order (some code casts these values to `int` in order to
     /// compare them against each other).
+    ScrollbarbuttonUp,
     ScrollbarbuttonDown,
     ScrollbarbuttonLeft,
     ScrollbarbuttonRight,
-    ScrollbarbuttonUp,
     /// The scrollbar thumb.
     ScrollbarthumbHorizontal,
     ScrollbarthumbVertical,

widget/windows/nsNativeThemeWin.cpp

@@ -1056,7 +1056,7 @@ nsNativeThemeWin::GetThemePartAndState(nsIFrame* aFrame, WidgetType aWidgetType,
     case StyleAppearance::ScrollbarbuttonLeft:
     case StyleAppearance::ScrollbarbuttonRight: {
       aPart = SP_BUTTON;
-      aState = (int(aWidgetType) - int(StyleAppearance::ScrollbarbuttonDown))*4;
+      aState = (int(aWidgetType) - int(StyleAppearance::ScrollbarbuttonUp))*4;
       EventStates eventState = GetContentState(aFrame, aWidgetType);
       if (!aFrame)
         aState += TS_NORMAL;
@@ -1071,7 +1071,7 @@ nsNativeThemeWin::GetThemePartAndState(nsIFrame* aFrame, WidgetType aWidgetType,
         else if (eventState.HasState(NS_EVENT_STATE_HOVER))
           aState += TS_HOVER;
         else if (parentState.HasState(NS_EVENT_STATE_HOVER))
-          aState = (int(aWidgetType) - int(StyleAppearance::ScrollbarbuttonDown)) + SP_BUTTON_IMPLICIT_HOVER_BASE;
+          aState = (int(aWidgetType) - int(StyleAppearance::ScrollbarbuttonUp)) + SP_BUTTON_IMPLICIT_HOVER_BASE;
         else
           aState += TS_NORMAL;
       }