style: Expose scrollbar-inline-size as a chrome-only environment variable

After the previous patches this is rather straight-forward.

Reviewed in: https://phabricator.services.mozilla.com/D168148

components/style/custom_properties.rs

@@ -85,6 +85,10 @@ fn get_content_preferred_color_scheme(_device: &Device) -> VariableValue {
     VariableValue::ident("light")
 }
 
+fn get_scrollbar_inline_size(device: &Device) -> VariableValue {
+    VariableValue::pixels(device.scrollbar_inline_size().px())
+}
+
 static ENVIRONMENT_VARIABLES: [EnvironmentVariable; 4] = [
     make_variable!(atom!("safe-area-inset-top"), get_safearea_inset_top),
     make_variable!(atom!("safe-area-inset-bottom"), get_safearea_inset_bottom),
@@ -120,7 +124,7 @@ macro_rules! lnf_int_variable {
     }};
 }
 
-static CHROME_ENVIRONMENT_VARIABLES: [EnvironmentVariable; 6] = [
+static CHROME_ENVIRONMENT_VARIABLES: [EnvironmentVariable; 7] = [
     lnf_int_variable!(
         atom!("-moz-gtk-csd-titlebar-radius"),
         TitlebarRadius,
@@ -146,6 +150,10 @@ static CHROME_ENVIRONMENT_VARIABLES: [EnvironmentVariable; 6] = [
         atom!("-moz-content-preferred-color-scheme"),
         get_content_preferred_color_scheme
     ),
+    make_variable!(
+        atom!("scrollbar-inline-size"),
+        get_scrollbar_inline_size
+    ),
 ];
 
 impl CssEnvironment {

components/style/gecko/media_queries.rs

@@ -199,6 +199,18 @@ impl Device {
         unsafe { bindings::Gecko_GetBaseSize(self.document(), language.as_ptr(), generic) }
     }
 
+    /// Gets the size of the scrollbar in CSS pixels.
+    pub fn scrollbar_inline_size(&self) -> Length {
+        let pc = match self.pres_context() {
+            Some(pc) => pc,
+            // XXX: we could have a more reasonable default perhaps.
+            None => return Length::new(0.0),
+        };
+        Length::new(unsafe {
+            bindings::Gecko_GetScrollbarInlineSize(pc)
+        })
+    }
+
     /// Queries font metrics
     pub fn query_font_metrics(
         &self,