Add missing cast in generated bindings

I accidentially used a ctor instead of the Cast operator in one place
dealing with PropertyCallbackInfo::This

BUG=v8:3274
R=haraken@chromium.org
TBR=haraken@chromium.org

Review URL: https://codereview.chromium.org/255763005

git-svn-id: svn://svn.chromium.org/blink/trunk@172630 bbb929c8-8fbe-4397-9dbb-9b2b20218538