[Tizen] Enable/Disable the shared process mode at runtime.

The shared process mode has been enabled by default, which makes
debugging very hard so we need to enable/disable it at runtime.

This patch allows to add a new command line option
(--disable-shared-process-mode) for disabling the shared process
mode at runtime.

Bug=XWALK-2987
1) const method
2) style: simple getter should be named like 'shared_process_mode_enabled'