Use Au instead of Length in flexbox code (#30704)

* convert border and padding to app units in flexbox

* convert margin to app units in flexbox

* cleanup, fmt

* add todo comment

* fmt

* add comment

* use Length instead of CSSPixelLength: they are same

components/layout_2020/flexbox/geom.rs

@@ -63,6 +63,16 @@ impl<T> FlexRelativeSides<T> {
             cross: self.cross_start + self.cross_end,
         }
     }
+
+    // TODO(#29819): Check if this function can be removed after we convert everything to Au.
+    pub fn map<U>(&self, f: impl Fn(&T) -> U) -> FlexRelativeSides<U> {
+        FlexRelativeSides {
+            main_start: f(&self.main_start),
+            main_end: f(&self.main_end),
+            cross_start: f(&self.cross_start),
+            cross_end: f(&self.cross_end),
+        }
+    }
 }
 
 /// One of the two bits set by the `flex-direction` property