In WebKit, we have a small integer cache because calling v8::Integer::New is
slow. This patch adds a faster API for creating integers that requires the
caller to supply the v8::Isolate, saving us the work of looking up the isolate
in thread-local storage.

BUG=

Review URL: https://codereview.chromium.org/11212004
Patch from Adam Barth <abarth@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12773 ce2b1a6d-e550-0410-aec6-3dcde31c8c00