Fix assertion scope in Runtime_GetScript.

The HeapIterator implies DisallowHeapAllocation, but Script::GetWrapper
may allocate.

LOG=N
R=jkummerow@chromium.org
BUG=chromium:410033

Review URL: https://codereview.chromium.org/680283002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25001 ce2b1a6d-e550-0410-aec6-3dcde31c8c00