Copy V8 attribute to nativeBridge

Since we wrapped the native bridge in commit b203462c, the V8 field
is not accessible on tabris._nativeBridge anymore. We need this field
to detect the V8 engine to enable optimizations in CanvasContext.

Change-Id: If4f7c0a7a38e6a0aeff7ad08be72d9d9fafc60f3