[WebDriver] Add support for shadow roots

[Carlos Garcia Campos]

See https://w3c.github.io/webdriver/#shadow-root, https://w3c.github.io/webdriver/#find-element-from-shadow-root, https://w3c.github.io/webdriver/#find-elements-from-shadow-root and https://w3c.github.io/webdriver/#get-element-shadow-root

imported/w3c/webdriver/tests/find_element_from_shadow_root/
imported/w3c/webdriver/tests/find_elements_from_shadow_root/
imported/w3c/webdriver/tests/get_element_shadow_root/

[Radar WebKit Bug Importer]

<rdar://problem/83667535>

[Carlos Garcia Campos]

Created attachment 450514 [details]
Patch

[Adrian Perez]

Comment on attachment 450514 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=450514&action=review

> Source/WebDriver/Session.cpp:1139
> +                if (isShadowRoot == ElementIsShadowRoot::Yes && result.errorString() == "stale element reference") {

Wouldn't it be more robust to compare the error code instead of comparing
strings? As in:

  result.errorCode == CommandResult::ErrorCode::StaleElementReference

> Source/WebDriver/WebDriverService.cpp:1461
>  }

These two functions are so similar... the only variation AFAICS is
using FindElementsMode::{Multiple,Single} in the last line; maybe most
of the function body could be factored out?

[Carlos Garcia Campos]

Committed r289202 (?): <https://commits.webkit.org/r289202>