style: Add 'row-gap' and 'gap' properties; make 'grid-[column|row]-gap' and 'grid-gap' alias the respective unprefixed properties.

This also makes 'normal' the initial value for the grid-*
properties, per:
https://github.com/w3c/csswg-drafts/issues/2294#issuecomment-369313438

Bug: 1398482
Reviewed-by: emilio

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

@@ -31,17 +31,6 @@ ${helpers.predefined_type(
 
 
 
-${helpers.predefined_type(
-    "column-gap",
-    "length::NonNegativeLengthOrPercentageOrNormal",
-    "Either::Second(Normal)",
-    extra_prefixes="moz",
-    servo_pref="layout.columns.enabled",
-    animation_value_type="NonNegativeLengthOrPercentageOrNormal",
-    spec="https://drafts.csswg.org/css-multicol/#propdef-column-gap",
-    servo_restyle_damage = "reflow",
-)}
-
 ${helpers.single_keyword("column-fill", "balance auto", extra_prefixes="moz",
                          products="gecko", animation_value_type="discrete",
                          spec="https://drafts.csswg.org/css-multicol/#propdef-column-fill")}

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

@@ -302,13 +302,6 @@ ${helpers.predefined_type("object-position",
                           animation_value_type="ComputedValue")}
 
 % for kind in ["row", "column"]:
-    ${helpers.predefined_type("grid-%s-gap" % kind,
-                              "NonNegativeLengthOrPercentage",
-                              "computed::NonNegativeLengthOrPercentage::zero()",
-                              spec="https://drafts.csswg.org/css-grid/#propdef-grid-%s-gap" % kind,
-                              animation_value_type="NonNegativeLengthOrPercentage",
-                              products="gecko")}
-
     % for range in ["start", "end"]:
         ${helpers.predefined_type("grid-%s-%s" % (kind, range),
                                   "GridLine",
@@ -355,3 +348,23 @@ ${helpers.predefined_type("grid-template-areas",
                           products="gecko",
                           animation_value_type="discrete",
                           spec="https://drafts.csswg.org/css-grid/#propdef-grid-template-areas")}
+
+${helpers.predefined_type("column-gap",
+                          "length::NonNegativeLengthOrPercentageOrNormal",
+                          "Either::Second(Normal)",
+                          alias="grid-column-gap",
+                          extra_prefixes="moz",
+                          servo_pref="layout.columns.enabled",
+                          spec="https://drafts.csswg.org/css-align-3/#propdef-column-gap",
+                          animation_value_type="NonNegativeLengthOrPercentageOrNormal",
+                          servo_restyle_damage = "reflow")}
+
+// no need for -moz- prefixed alias for this property
+${helpers.predefined_type("row-gap",
+                          "length::NonNegativeLengthOrPercentageOrNormal",
+                          "Either::Second(Normal)",
+                          alias="grid-row-gap",
+                          servo_pref="layout.columns.enabled",
+                          spec="https://drafts.csswg.org/css-align-3/#propdef-row-gap",
+                          animation_value_type="NonNegativeLengthOrPercentageOrNormal",
+                          servo_restyle_damage = "reflow")}

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

@@ -108,30 +108,30 @@
     }
 </%helpers:shorthand>
 
-<%helpers:shorthand name="grid-gap" sub_properties="grid-row-gap grid-column-gap"
+<%helpers:shorthand name="gap" alias="grid-gap" sub_properties="row-gap column-gap"
-                    spec="https://drafts.csswg.org/css-grid/#propdef-grid-gap"
+                    spec="https://drafts.csswg.org/css-align-3/#gap-shorthand"
                     products="gecko">
-  use properties::longhands::{grid_row_gap, grid_column_gap};
+  use properties::longhands::{row_gap, column_gap};
 
   pub fn parse_value<'i, 't>(context: &ParserContext, input: &mut Parser<'i, 't>)
                              -> Result<Longhands, ParseError<'i>> {
-      let row_gap = grid_row_gap::parse(context, input)?;
+      let r_gap = row_gap::parse(context, input)?;
-      let column_gap = input.try(|input| grid_column_gap::parse(context, input)).unwrap_or(row_gap.clone());
+      let c_gap = input.try(|input| column_gap::parse(context, input)).unwrap_or(r_gap.clone());
 
       Ok(expanded! {
-        grid_row_gap: row_gap,
+        row_gap: r_gap,
-        grid_column_gap: column_gap,
+        column_gap: c_gap,
       })
   }
 
   impl<'a> ToCss for LonghandsToSerialize<'a>  {
       fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result where W: fmt::Write {
-          if self.grid_row_gap == self.grid_column_gap {
+          if self.row_gap == self.column_gap {
-            self.grid_row_gap.to_css(dest)
+            self.row_gap.to_css(dest)
           } else {
-            self.grid_row_gap.to_css(dest)?;
+            self.row_gap.to_css(dest)?;
             dest.write_str(" ")?;
-            self.grid_column_gap.to_css(dest)
+            self.column_gap.to_css(dest)
           }
       }
   }