Fix NaCl-related build errors in MemorySanitizer builds.

- Do not try to include sanitizer headers if defined(OS_NACL) - they are not
available.
- Do not use -Wl,z,defs when building the pepper plugin.

BUG=341917
TBR=bradnelson@chromium.org,thakis@chromium.org

Review URL: https://codereview.chromium.org/133813007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249739 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249739