Dispatch selectionchange event per element

According to the new spec[1], we need enqueue selectionchange event
per element. WebKit also updated the implementation of selectionchange
event in webcore[2].

[1] w3c/selection-api#172
[2] https://commits.webkit.org/276238@main

Change-Id: If2020febecdf32d6165023712442f16c6e91bd4c

selection/onselectionchange-on-distinct-text-controls.html

@@ -22,7 +22,7 @@
     assert_equals(selectionChangeCount1, 0);
     assert_equals(selectionChangeCount2, 0);
     await new Promise(setTimeout);
-    assert_equals(selectionChangeCount1, 1);
+    assert_equals(selectionChangeCount1, 2);
     assert_equals(selectionChangeCount2, 1);
   })();
 }, "selectionchange event on each input element fires independently");
@@ -39,7 +39,7 @@
     assert_equals(selectionChangeCount1, 0);
     assert_equals(selectionChangeCount2, 0);
     await new Promise(setTimeout);
-    assert_equals(selectionChangeCount1, 1);
+    assert_equals(selectionChangeCount1, 2);
     assert_equals(selectionChangeCount2, 1);
   })();
 }, "selectionchange event on each textarea element fires independently");