RuntimeContext must be destructed explicitly

When MessageLoop is stopped, runtime_context_ must be set to null.
It will call RuntimeContext destruction method, which happened on
MainThread. It blocked download_browsertest.

BUG=https://crosswalk-project.org/jira/browse/XWALK-408
Merge pull request #1219 from yinli/runtime_context_destruction

RuntimeContext must be destructed explicitly