DevTools: initialize "format script" button with current UISourceCode

As of today, the "format script" button is initialized as hidden and
will update its state in response to editorSelected event. However,
in case the event is sent prior to button construction, it should be
initialized with current selected editor.

BUG=424089
R=vsevik

Review URL: https://codereview.chromium.org/658293003

git-svn-id: svn://svn.chromium.org/blink/trunk@183881 bbb929c8-8fbe-4397-9dbb-9b2b20218538