Generate valid form for {background|mask}-position from their longhands.

If there is any longhand consisting of both keyword and position, we
should attach both keyword and position for both longhands when serialize
them.

1. To generate a serialized result with keyword, we add a macro function
`to_css_with_keyword` accepted HorizontalPosition and VerticalPosition as
the parameter to finish that. For the longhands which missing keyword, we
use the default keyword.
(`left` for `*-position-x` and `top` for `*-position-y`)

2. Update `Position::to_css` to generate the valid format by calling
`to_css_with_keyword` with HorizontalPosition and VerticalPosition.

3. Update `to_css` to use new `Position::to_css` in background,
background-position, mask and mask-position.

MozReview-Commit-ID: 5Bnhdsi5yeM

components/style/properties/shorthand/background.mako.rs

@@ -159,11 +159,17 @@
                 }
 
                 try!(image.to_css(dest));
-                % for name in "repeat attachment position_x position_y".split():
+                % for name in "repeat attachment".split():
                     try!(write!(dest, " "));
                     try!(${name}.to_css(dest));
                 % endfor
 
+                try!(write!(dest, " "));
+                Position {
+                    horizontal: position_x.clone(),
+                    vertical: position_y.clone()
+                }.to_css(dest)?;
+
                 if *size != background_size::single_value::get_initial_specified_value() {
                     try!(write!(dest, " / "));
                     try!(size.to_css(dest));
@@ -225,9 +231,11 @@
                 return Ok(());
             }
             for i in 0..len {
-                self.background_position_x.0[i].to_css(dest)?;
-                dest.write_str(" ")?;
-                self.background_position_y.0[i].to_css(dest)?;
+                Position {
+                    horizontal: self.background_position_x.0[i].clone(),
+                    vertical: self.background_position_y.0[i].clone()
+                }.to_css(dest)?;
+
                 if i < len - 1 {
                     dest.write_str(", ")?;
                 }

components/style/properties/shorthand/mask.mako.rs

@@ -148,9 +148,12 @@
                 dest.write_str(" ")?;
                 mode.to_css(dest)?;
                 dest.write_str(" ")?;
-                position_x.to_css(dest)?;
-                dest.write_str(" ")?;
-                position_y.to_css(dest)?;
+
+                Position {
+                    horizontal: position_x.clone(),
+                    vertical: position_y.clone()
+                }.to_css(dest)?;
+
                 if *size != mask_size::single_value::get_initial_specified_value() {
                     dest.write_str(" / ")?;
                     size.to_css(dest)?;
@@ -218,9 +221,11 @@
             }
 
             for i in 0..len {
-                self.mask_position_x.0[i].to_css(dest)?;
-                dest.write_str(" ")?;
-                self.mask_position_y.0[i].to_css(dest)?;
+                Position {
+                    horizontal: self.mask_position_x.0[i].clone(),
+                    vertical: self.mask_position_y.0[i].clone()
+                }.to_css(dest)?;
+
                 if i < len - 1 {
                     dest.write_str(", ")?;
                 }

components/style/values/specified/position.rs

@@ -32,27 +32,36 @@ pub struct Position {
 
 impl ToCss for Position {
     fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-        let mut space_present = false;
-        if let Some(horiz_key) = self.horizontal.keyword {
-            try!(horiz_key.to_css(dest));
-            try!(dest.write_str(" "));
-            space_present = true;
-        };
-        if let Some(ref horiz_pos) = self.horizontal.position {
-            try!(horiz_pos.to_css(dest));
-            try!(dest.write_str(" "));
-            space_present = true;
-        };
-        if let Some(vert_key) = self.vertical.keyword {
-            try!(vert_key.to_css(dest));
-            space_present = false;
-        };
-        if let Some(ref vert_pos) = self.vertical.position {
-            if space_present == false {
-                try!(dest.write_str(" "));
-            }
-            try!(vert_pos.to_css(dest));
-        };
+        macro_rules! to_css_with_keyword {
+            ($pos:expr, $default_keyword:expr) => {
+                $pos.keyword.unwrap_or($default_keyword).to_css(dest)?;
+
+                if let Some(ref position) = $pos.position {
+                    dest.write_str(" ")?;
+                    position.to_css(dest)?;
+                };
+            };
+        }
+
+        let mut is_keyword_needed = false;
+        let position_x = &self.horizontal;
+        let position_y = &self.vertical;
+
+        if (position_x.keyword.is_some() && position_x.position.is_some()) ||
+           (position_y.keyword.is_some() && position_y.position.is_some()) {
+            is_keyword_needed = true;
+        }
+
+        if is_keyword_needed {
+            to_css_with_keyword!(position_x, Keyword::Left);
+            dest.write_str(" ")?;
+            to_css_with_keyword!(position_y, Keyword::Top);
+        } else {
+            position_x.to_css(dest)?;
+            dest.write_str(" ")?;
+            position_y.to_css(dest)?;
+        }
+
         Ok(())
     }
 }
@@ -403,6 +412,7 @@ impl ToCss for VerticalPosition {
         };
         Ok(())
     }
+
 }
 
 impl Parse for VerticalPosition {