Tag Mac OS X VM regions allocated by V8. This has no effect other than making
it possible to identify V8 allocations in the vmmap(1) memory profiling tool,
to get a better idea of an embedder's/Chrome's memory usage.
CRBUG=23455
TEST=Tested by running vmmap(1) and looking for "Memory tag=255" in the output.

Review URL: http://codereview.chromium.org/244051

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3001 ce2b1a6d-e550-0410-aec6-3dcde31c8c00