Auto merge of #10034 - rilut:implement-elementsfrompoint, r=emilio

Implement Document#elementsFromPoint

Fixes #9859.

I'm trying to implement Document#elementsFromPoint, which I need to reuse the `get_nodes_under_mouse` and `mouse_over` function which have been removed a days ago in #9715. So I added it back while I'm not sure if my implementation is correct. Any advice will be greatly appreciated.

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tests/wpt/include.ini

@@ -55,3 +55,5 @@ skip: true
   skip: false
 [WebIDL]
   skip: false
+[cssom-view]
+  skip: false

tests/wpt/metadata/cssom-view/elementsFromPoint.html.ini

@@ -0,0 +1,17 @@
+[elementsFromPoint.html]
+  type: testharness
+  [co-ordinates larger than the viewport from in iframe]
+    expected: FAIL
+
+  [SVG element at x,y]
+    expected: FAIL
+
+  [transformed element at x,y]
+    expected: FAIL
+
+  [no hit target at x,y]
+    expected: FAIL
+
+  [No viewport available]
+    expected: FAIL
+

tests/wpt/metadata/cssom-view/scrollingElement.html.ini

@@ -0,0 +1,5 @@
+[scrollingElement.html]
+  type: testharness
+  [Tests for scrollingElement]
+    expected: FAIL
+