Disable source fortification under Clang on Linux.
Right now source fortification isn't supported by Clang (http://llvm.org/bugs/show_bug.cgi?id=16821)
It may also prevent the sanitizer tools from catching some bugs.

BUG=162783,341809
R=hans@chromium.org

Review URL: https://codereview.chromium.org/138913015

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251868 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251868