Auto merge of #6662 - tschneidereit:client-geometry, r=glennw,pcwatson

Implement Element.client{Top,Left,Width,Height}

This isn't done, but contains a working implementation of at least `clientTop`. Feedback would be much appreciated: it's probably far from ideal.

Implementing `clientLeft` is straight-forward, I think, but `clientWidth` and `clientHeight` require accessing the `border_box` - and I don't know how that works, yet.

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components/script/dom/element.rs

@@ -1317,6 +1317,26 @@ impl<'a> ElementMethods for &'a Element {
             rect.origin.x + rect.size.width)
     }
 
+    fn ClientTop(self) -> i32 {
+        let node = NodeCast::from_ref(self);
+        node.get_client_rect().origin.y
+    }
+
+    fn ClientLeft(self) -> i32 {
+        let node = NodeCast::from_ref(self);
+        node.get_client_rect().origin.x
+    }
+
+    fn ClientWidth(self) -> i32 {
+        let node = NodeCast::from_ref(self);
+        node.get_client_rect().size.width
+    }
+
+    fn ClientHeight(self) -> i32 {
+        let node = NodeCast::from_ref(self);
+        node.get_client_rect().size.height
+    }
+
     // https://dvcs.w3.org/hg/innerhtml/raw-file/tip/index.html#widl-Element-innerHTML
     fn GetInnerHTML(self) -> Fallible<DOMString> {
         //XXX TODO: XML case