Bug 921586: Avoid 'may be used uninitialized' warnings in StructuredClone.cpp. r=njn

Specifically, it seems that G++ 4.8.1 is unable to tell that the call to
AutoCompartment::~AutoCompartment is protected by the Maybe::constructed
flag.

The "solution" is not to use Maybe here; although Debugger.cpp uses
Maybe<AutoCompartment> everywhere, it gets away with it because it always
actually constructs the Maybe(?!?).

It's a shame to let the compiler's stupidity shape the code, but that
particular warning is valuable elsewhere, so I feel that we shouldn't just
disable the warning.

In file included from /home/jimb/moz/dbg/js/src/jsobjinlines.h:20:0,
                 from /home/jimb/moz/dbg/js/src/vm/StructuredClone.cpp:44:
/home/jimb/moz/dbg/js/src/jscompartmentinlines.h: In function ‘bool JS_StructuredClone(JSContext*, JS::Value, JS::Value*, const JSStructuredCloneCallbacks*, void*)’:
/home/jimb/moz/dbg/js/src/jscompartmentinlines.h:45:35: warning: ‘ac.js::AutoCompartment::origin_’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     cx_->leaveCompartment(origin_);
                                   ^
/home/jimb/moz/dbg/js/src/vm/StructuredClone.cpp:1521:41: note: ‘ac.js::AutoCompartment::origin_’ was declared here
         mozilla::Maybe<AutoCompartment> ac;
                                         ^
In file included from /home/jimb/moz/dbg/js/src/jsobjinlines.h:20:0,
                 from /home/jimb/moz/dbg/js/src/vm/StructuredClone.cpp:44:
/home/jimb/moz/dbg/js/src/jscompartmentinlines.h:45:35: warning: ‘ac.js::AutoCompartment::cx_’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     cx_->leaveCompartment(origin_);
                                   ^
/home/jimb/moz/dbg/js/src/vm/StructuredClone.cpp:1521:41: note: ‘ac.js::AutoCompartment::cx_’ was declared here
         mozilla::Maybe<AutoCompartment> ac;
                                         ^