Reviewed by Pavel Feldman.

        Add separate method for pause command to WebDevToolsAgent
        and WebDevToolsFrontendClient interfaces so that pause command
        can be processed on the IO thread.

        https://bugs.webkit.org/show_bug.cgi?id=31934

        * public/WebDevToolsAgent.h:
        * public/WebDevToolsFrontendClient.h:
        (WebKit::WebDevToolsFrontendClient::sendDebuggerCommandToAgent):
        (WebKit::WebDevToolsFrontendClient::sendDebuggerPauseScript):
        (WebKit::WebDevToolsFrontendClient::activateWindow):
        (WebKit::WebDevToolsFrontendClient::closeWindow):
        (WebKit::WebDevToolsFrontendClient::dockWindow):
        (WebKit::WebDevToolsFrontendClient::undockWindow):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@51456 268f45cc-cd09-0410-ab3c-d52691b4dbfc