This gives us dynamic loading support in the Mac sandbox.

It will allow us to address a TODO in renderer_sandbox_support_linux.h.

BUG=http://code.google.com/p/nativeclient/issues/detail?id=469
BUG=http://code.google.com/p/nativeclient/issues/detail?id=583
TEST=nacl_ui_tests

Review URL: http://codereview.chromium.org/4807003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@66434 0039d316-1c4b-4281-b951-d872f2087c98