Improve performance of v8 string hashes by using symbol id

Improve performance of v8 string hashes by using symbol id

The symbol id is a unique integer associated with a string symbol in
V8.  If two symbol ids are equal, then the strings are also equal.
However, if the two symbol ids are not equal, the strings may still
be equal.

Change-Id: Ic28f51a1eba568ae4b2a054e278f6e5e454725d6
Reviewed-on: http://codereview.qt.nokia.com/990
Reviewed-by: Aaron Kennedy <aaron.kennedy@nokia.com>