WebDriver: correctly handle main frame handles

WebDriver: correctly handle main frame handles
https://bugs.webkit.org/show_bug.cgi?id=174668

Reviewed by Brian Burg.

When I switched to use std::optional instead of empty strings for the browsing contexts in WebDriver, I forgot
that automation uses empty string for frames to refer to the main frame. We should handle that case, because we
are currently considering empty strings as valid browsing context. It's not a big deal because Automation
converts back the empty string received to the main frame, though. We should also ensure we close the current
browsing context when switching to a new top level browsing context. This patch adds to helper private methods
to switch browsing contexts that deal with the special cases.

* Session.cpp:
(WebDriver::Session::close):
(WebDriver::Session::switchToTopLevelBrowsingContext):
(WebDriver::Session::switchToBrowsingContext):
(WebDriver::Session::createTopLevelBrowsingContext):
(WebDriver::Session::go):
(WebDriver::Session::back):
(WebDriver::Session::forward):
(WebDriver::Session::refresh):
(WebDriver::Session::switchToWindow):
(WebDriver::Session::switchToFrame):
(WebDriver::Session::switchToParentFrame):
* Session.h:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@219721 268f45cc-cd09-0410-ab3c-d52691b4dbfc