initialize result variables in v8 bindings

https://codereview.chromium.org/360703003 and
https://codereview.chromium.org/362993004 can falsely trigger
use of uninitialized variable warnings in some toolchains,
for integer "return" variables.  Let's explicitly initialize
these variables.

BUG=341031

Review URL: https://codereview.chromium.org/390223004

git-svn-id: svn://svn.chromium.org/blink/trunk@178263 bbb929c8-8fbe-4397-9dbb-9b2b20218538