style: Add an input-region-margin to widgets, and implement it on Linux

Recompute the input region when resizing the widget and so on, and use
it to check for rollups.

Depends on D148211

Differential Revision: https://phabricator.services.mozilla.com/D148222

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

@@ -98,6 +98,16 @@ ${helpers.predefined_type(
     enabled_in="chrome",
 )}
 
+${helpers.predefined_type(
+    "-moz-window-input-region-margin",
+    "Length",
+    "computed::Length::zero()",
+    engines="gecko",
+    animation_value_type="ComputedValue",
+    spec="None (Nonstandard internal property)",
+    enabled_in="chrome",
+)}
+
 // TODO(emilio): Probably also should be hidden from content.
 ${helpers.predefined_type(
     "-moz-force-broken-image-icon",