Remove runtime context from 'XWalkBrowserMainParts' class

SSIA. Runtime context should be obtained from 'XWalkRunner' - there is
no need to store it in "main parts" and have a runtime context getter
there.
Merge pull request #1663 from pozdnyakov/Remove_runtime_context_from_main_parts

Remove runtime context from 'XWalkBrowserMainParts' class