WIP: (not yet compiled) Add SharedMem support for CefString.

Reimplement std::string support in terms of CefString: when we shm_write()
either std::string or CefString to shared memory, enable reading as either
CefString or std::string. But store as a sequence of CefString::char_type so
the reading side can leverage CefString's ability to directly attach to a
block of string data without copying.

Change JSInfo (a struct used solely for serializing to/from shared memory) to
store CefString fields rather than std::string, so we can leverage the
aforementioned CefString optimization.