Implement Get Element Shadow Root
https://www.w3.org/TR/webdriver2/#dfn-get-element-shadow-root
cc: @xiaochengh, @yezhizhen, @PotatoCP
Testing:
`\tests\wpt\tests\webdriver\tests\classic\get_element_shadow_root\get.py`
is blocked by `no_browsing_context` and `closed_window`
pass for other sub-tests.
Signed-off-by: batu_hoang <longvatrong111@gmail.com>