Annotate the libc overrides in zygote_main_linux.cc for MemorySanitizer.

BUG=123263
TBR=jln@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/169363006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251683 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251683