Make v8 compilable without V8_USE_UNSAFE_HANDLES.

Make v8 compilable without V8_USE_UNSAFE_HANDLES.

Without this modification, we get this error: "dereferencing type-punned pointer
will break strict-aliasing rules" (GCC strict aliasing).

Also included small CcTest sanity fixes: isolate() cannot return anything else
than default_isolate().

BUG=
TBR=dcarney@chromium.org

Review URL: https://codereview.chromium.org/14894006

Patch from Marja Hölttä <marja@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14554 ce2b1a6d-e550-0410-aec6-3dcde31c8c00