MEMORY_SANITIZER is defined by default in certain compiler environments (when
MemorySanitizer is enabled, of course). Do not redefine it in that case.

R=danno@chromium.org

Review URL: https://codereview.chromium.org/34503012

Patch from Evgeniy Stepanov <eugenis@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@17344 ce2b1a6d-e550-0410-aec6-3dcde31c8c00