Resolve a slot element's directionality from the flattened tree

[chromium-wpt-export-bot]

This CL applies the new agreements for a <slot> element 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] https://github.com/whatwg/html/issues/7299 [2] https://github.com/whatwg/html/issues/3699#issuecomment-988890095 [3] https://github.com/web-platform-tests/wpt/pull/29820

Bug: 1236384 Change-Id: I43ca7351fc4d65dd4f72fe4f9627ffa5382e5c20 Reviewed-on: https://chromium-review.googlesource.com/3343556 WPT-Export-Revision: cec8ee092ba6e5b2efb580e28823a640d50b707e