Return descriptor ownership after TrimDescriptorArray to allow resharing of descriptor arrays

BUG=None
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/27197002

Patch from Ross McIlroy <rmcilroy@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17237 ce2b1a6d-e550-0410-aec6-3dcde31c8c00