mac: Let `mac-tool flock` exit with the exit code of the child program.
mac: Let `mac-tool flock` exit with the exit code of the child program.

TEST=When a link fails in the mac/ninja build, postbuilds no longer try to run.
Review URL: https://chromiumcodereview.appspot.com/9407030

csulmone/gyp,csulmone/gyp,csulmone/gyp,csulmone/gyp