Bootstrapper::DetachGlobal also need to unset global_proxy's constructor to remove all refs to context

DetachGlobal detaches original context of a global proxy object.

Before this patch, the constructor JSFunction still carried a reference to the old context after |Bootstrapper::DetachGlobal| call.
This patch removes the reference by setting the constructor null.

TEST=http/tests/security/isolatedWorld w/ --enable-leak-detection
LOG=N
BUG=364377
R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/397953009

Patch from Kouhei Ueno <kouhei@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22460 ce2b1a6d-e550-0410-aec6-3dcde31c8c00