Metal: Invert CW and CCW because we compile with flip Y

BUG=dawn:43

Change-Id: I6d09118d141046cd307604b1b4d53a82d8d15bf6
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/6761
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/dawn_native/metal/RenderPipelineMTL.mm

@@ -283,10 +283,11 @@ namespace dawn_native { namespace metal {
         }
 
         MTLWinding MTLFrontFace(dawn::FrontFace face) {
+            // Note that these are inverted because we flip the Y coordinate in the vertex shader
             switch (face) {
-                case dawn::FrontFace::CCW:
-                    return MTLWindingCounterClockwise;
                 case dawn::FrontFace::CW:
+                    return MTLWindingCounterClockwise;
+                case dawn::FrontFace::CCW:
                     return MTLWindingClockwise;
             }
         }