Mark mjsunit/setters-on-elements as flaky

TBR=machenbach@chromium.org
LOG=n
BUG=v8:3784
NOTREECHECKS=true

Review URL: https://codereview.chromium.org/817293002

Cr-Commit-Position: refs/heads/master@{#25929}

test/mjsunit/mjsunit.status

@@ -51,6 +51,9 @@
   # Issue 3389: deopt_every_n_garbage_collections is unsafe
   'regress/regress-2653': [SKIP],
 
+  # Issue 3784: setters-on-elements is flaky
+  'setters-on-elements': [PASS, FAIL],
+
   ##############################################################################
   # TurboFan compiler failures.