Run nacl_loader_unittests on trybots and waterfall bots

This test does nothing on Mac or Windows, so we do not add
this to them.

TEST=trybot
BUG=364751

Review URL: https://codereview.chromium.org/253273002

git-svn-id: http://src.chromium.org/chrome/trunk/src/testing@268520 4ff67af0-8c30-449e-8e8b-ad334ec8d88c