WebDriver: Set Cookie endpoint does not correctly set subdomain cookies

[Devin Rousso]

```py
from selenium import webdriver

driver = webdriver.Safari(executable_path='$build/Release/safaridriver')

driver.get("https://apple.com")
driver.add_cookie({
    'name': 'testfromotherdomain',
    'value': '42',
    'domain': 'school.apple.com',
})

driver.get("https://school.apple.com")
driver.add_cookie({
    'name': 'testfromsamedomain',
    'value': '42',
    'domain': 'school.apple.com',
})
```

After running the above, stop the active session and inspect <https://school.apple.com>.  There should be two cookies ("testfromotherdomain" and "testfromsamedomain"), but there's only one ("testfromsamedomain").

[Devin Rousso]

<rdar://problem/49233240>

[Devin Rousso]

Created attachment 367336 [details]
Patch

[Joseph Pecoraro]

Comment on attachment 367336 [details]
Patch

r=me

[WebKit Commit Bot]

Comment on attachment 367336 [details]
Patch

Clearing flags on attachment: 367336

Committed r244281: <https://trac.webkit.org/changeset/244281>

[WebKit Commit Bot]

All reviewed patches have been landed.  Closing bug.