Add IsGeneratorFunction and IsGeneratorObject checks to v8::Value.

R=ulan@chromium.org, yangguo@chromium.org, wingo, yangguo

Review URL: https://codereview.chromium.org/608503002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24278 ce2b1a6d-e550-0410-aec6-3dcde31c8c00