Modify JS Object to QVariant conversion

Modify JS Object to QVariant conversion

Previously, JS Objects would be converted to a QVariantMap where
each value in the map was a QVariant from toVariant(propertyValue).
Unfortunately, this would result in a crash if the object had a
reference to another object which had a reference to the original
object, due to the circular reference.

This commit changes the conversion code to use
QV8Engine::variantMapFromJS() instead, which avoids cyclic references.

Task-number: QTBUG-21626
Change-Id: I129048c8704ae0d1095a02d0ce4c0fe5850b1b20
Reviewed-on: http://codereview.qt-project.org/5490
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Aaron Kennedy <aaron.kennedy@nokia.com>