[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 global Browser Context through XWalkRunner, so don't need this
    class member anymore.

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