[Sheriff] Mark failing test for nacl port.

BUG=v8:2978
R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/50333005

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@17445 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

test/mjsunit/mjsunit.status

@@ -259,6 +259,9 @@
 
   # Skip long running test that times out in debug mode and goes OOM on NaCl.
   'regress/regress-crbug-160010': [SKIP],
+
+  # Bug(v8:2978).
+  'lithium/MathExp': [PASS, FAIL],
 }],  # 'arch == nacl_ia32 or arch == nacl_x64'
 
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