Reland r22460 - "Bootstrapper::DetachGlobal also need to unset g..."

Reland r22460 - "Bootstrapper::DetachGlobal also need to unset g..."

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

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

Committed: https://code.google.com/p/v8/source/detail?r=22633

[jochen (gone - plz use gerrit), 2014-07-28 09:15:16]

Committed patchset #1 manually as r22633.