Better fix for MemoryChunk::owner().

Pointer arithmetic such as "owner_ - kFailureTag" is undefined behaviour
unless owner_ points to a valid object.

This allowed Clang to assume the subtraction would never be NULL,
causing problems in the caller (see https://codereview.chromium.org/12090072/).

To fix this, we should cast owner_ to intptr_t before doing the
arithmetic.

Review URL: https://codereview.chromium.org/12096089
Patch from Hans Wennborg <hans@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13570 ce2b1a6d-e550-0410-aec6-3dcde31c8c00