WebDriver: crash in Session::computeElementLayout when called without a current brows...

WebDriver: crash in Session::computeElementLayout when called without a current browsing context
https://bugs.webkit.org/show_bug.cgi?id=179917

Reviewed by Darin Adler.

In the case of computeElementLayout message, the frameHandle parameter is not optional, but we still need to
provide a valid value (empty string means the default frame) when m_currentBrowsingContext is std::nullopt. The
same applies to selectOptionElement.

* Session.cpp:
(WebDriver::Session::computeElementLayout):
(WebDriver::Session::selectOptionElement):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@225082 268f45cc-cd09-0410-ab3c-d52691b4dbfc