style: Implement -webkit-line-clamp.

Differential Revision: https://phabricator.services.mozilla.com/D20115

components/style/properties/data.py

@@ -337,6 +337,7 @@ class Longhand(object):
                 "OverflowWrap",
                 "OverscrollBehavior",
                 "Percentage",
+                "PositiveIntegerOrNone",
                 "Resize",
                 "SVGOpacity",
                 "SVGPaintOrder",

components/style/properties/gecko.mako.rs

@@ -2515,7 +2515,7 @@ fn static_assert() {
                           rotate scroll-snap-points-x scroll-snap-points-y
                           scroll-snap-coordinate -moz-binding will-change
                           offset-path shape-outside
-                          translate scale""" %>
+                          translate scale -webkit-line-clamp""" %>
 <%self:impl_trait style_struct_name="Box" skip_longhands="${skip_box_longhands}">
     #[inline]
     pub fn generate_combined_transform(&mut self) {
@@ -2924,6 +2924,27 @@ fn static_assert() {
         self.copy_offset_path_from(other);
     }
 
+    #[allow(non_snake_case)]
+    pub fn set__webkit_line_clamp(&mut self, v: longhands::_webkit_line_clamp::computed_value::T) {
+        self.gecko.mLineClamp = match v {
+            Either::First(n) => n.0 as u32,
+            Either::Second(None_) => 0,
+        };
+    }
+
+    ${impl_simple_copy('_webkit_line_clamp', 'mLineClamp')}
+
+    #[allow(non_snake_case)]
+    pub fn clone__webkit_line_clamp(&self) -> longhands::_webkit_line_clamp::computed_value::T {
+        match self.gecko.mLineClamp {
+            0 => Either::Second(None_),
+            n => {
+                debug_assert!(n <= std::i32::MAX as u32);
+                Either::First((n as i32).into())
+            }
+        }
+    }
+
 </%self:impl_trait>
 
 <%def name="simple_image_array_property(name, shorthand, field_name)">

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

@@ -670,3 +670,15 @@ ${helpers.predefined_type(
     animation_value_type="discrete",
     spec="https://compat.spec.whatwg.org/#touch-action",
 )}
+
+// Note that we only implement -webkit-line-clamp as a single, longhand
+// property for now, but the spec defines line-clamp as a shorthand for separate
+// max-lines, block-ellipsis, and continue properties.
+${helpers.predefined_type(
+    "-webkit-line-clamp",
+    "PositiveIntegerOrNone",
+    "Either::Second(None_)",
+    gecko_pref="layout.css.webkit-line-clamp.enabled",
+    animation_value_type="Integer",
+    spec="https://drafts.csswg.org/css-overflow-3/#line-clamp",
+)}