[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 init "browser_context_"
through "xwalk_runner_" in constructor function to make sure it's valid
at least anytime.

BUG=https://crosswalk-project.org/jira/browse/XWALK-3178
Merge pull request #2725 from wuyuehang/XWALK-3187

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