Auto merge of #16164 - heycam:stale-styles, r=bholley

stylo: Add argument to Servo_ResolveStyle to allow stale styles to be returned.

This is the Servo-side part of https://bugzilla.mozilla.org/show_bug.cgi?id=1350671.

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components/style/gecko_bindings/bindings.rs

@@ -1812,7 +1812,8 @@ extern "C" {
 }
 extern "C" {
     pub fn Servo_ResolveStyle(element: RawGeckoElementBorrowed,
-                              set: RawServoStyleSetBorrowed)
+                              set: RawServoStyleSetBorrowed,
+                              allow_stale: bool)
      -> ServoComputedValuesStrong;
 }
 extern "C" {