[Shared Runtime Process Model] Class skeleton added.
According to the discussion and the new design for the shared runtime
process model, the previous pull request (#750) is abandoned. This patch
is the first step towards the new design goal.  A new switch --daemon is
added to let xwalk go into daemon mode.  Loading a simple web page from
command line is still okay.
Please refer to
(https://docs.google.com/a/intel.com/document/d/1q2Zl_P949NLe5YnQoLP2OWcZStVJgXyHKZDUwhiUxJQ/edit\#\)
for more design details.