[Tizen] xwalk crashed when requests "json/new" in devtools

        The main reason is:
        The intialization of "XWalkContentBrowserClient::browser_context_" depends on
        invoking "XWalkContentBrowserClient::CreateRequestContext", so in this case when
        "GetDevToolsManagerDelegate" is creating, the "browser_context_" is still NULL.
        And we can obtain valid "browser_context_" through "xwalk_runner_".

        BUG=https://crosswalk-project.org/jira/browse/XWALK-3178