[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

@pozdnyakov @wang16 @takethathe please help to review it, thanks :)
@crosswalk-trybot 
I agree that init from constructor func will make browser_context_ has a valid value at least. But frankly speaking, I'm not very sure whether other scenes involed in creating new webcontents or runtimes will trigger "CreateRequestContext()" or not. So I can't judge if the re-assgin is unnecessary.