Resolve a slot element's directionality from the flattened tree

This CL applies the new agreements for a <slot> elelement on the
standards meeting[1][2].
 - Resolve the slot element's directionality from the flattened tree
 - Apply one exception that the slot element's directionality should
   be the same as a shadow host's directionality if the slot element
   has a dir=auto attribute and has no slotted content.

This CL adds the unit tests on blink and WPT tests will be added on [3].

[1] whatwg/html#7299
[2] whatwg/html#3699 (comment)
[3] #29820

Bug: 1236384
Change-Id: I43ca7351fc4d65dd4f72fe4f9627ffa5382e5c20

html/dom/elements/global-attributes/dir-slots-directionality.tentative.html

@@ -31,7 +31,7 @@
 test(() => {
   assert_equals(getComputedStyle(host1.firstChild).direction, "rtl");
   assert_equals(getComputedStyle(root2.querySelector("span")).direction, "rtl");
-  assert_equals(getComputedStyle(host3).direction, "ltr");
+  assert_equals(getComputedStyle(host3).direction, "rtl");
   assert_equals(getComputedStyle(root4.querySelector("span")).direction, "rtl");
   assert_equals(getComputedStyle(root5.querySelector("span")).direction, "rtl");
 }, 'Slots: Directionality');