Use object identity to detect cycles in JS-to-C++ type conversion

Use object identity to detect cycles in JS-to-C++ type conversion

The documentation for v8::Object::GetIdentityHash() states that the
hash value is not guaranteed to be unique (the current implementation
just returns a random number). Hence, the hash value should not be
used to determine whether an object has already been visited during
type conversion; in the worst (and non-deterministic) case, the
conversion will be "cut off" prematurely (due to identical hash
values for two different objects), resulting in data loss.

Instead, represent the visited objects as a set of V8 object handles.
This is safe since the type conversion is always done on the stack,
within a handle scope. Use v8::Object::GetIdentityHash() merely to
implement the qHash() specialization needed for the set.

V8 already provides an operator==() for handles, and it is documented
to return true "if the objects to which they refer are identical",
which is the behavior required by the set implementation.

Task-number: QTBUG-21681
Change-Id: I1f2a1eee8f7c197c02c2ffeaaa1fc0274e8ab740
Reviewed-by: Michael Brasser <michael.brasser@nokia.com>
Reviewed-by: Simon Hausmann <simon.hausmann@nokia.com>