Disable regress-crbug-518748 on debug

Seems to fail only on V8 Linux - debug - code serializer
(See http://build.chromium.org/p/client.v8/builders/V8%20Linux%20-%20debug%20-%20code%20serializer/builds/3762)

BUG=chromium:518748
TBR=mstarzinger@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/1294163003 .

Cr-Commit-Position: refs/heads/master@{#30233}

test/mjsunit/mjsunit.status

@@ -256,6 +256,9 @@
   # BUG(v8:4378).
   'regress/regress-crbug-501711': [PASS, ['isolates', SKIP]],
   'regress/regress-4279': [PASS, ['isolates', SKIP]],
+
+  # BUG(chromium:518748)
+  'regress/regress-crbug-518748': [PASS, ['mode == debug', SKIP]],
 }],  # ALWAYS
 
 ['novfp3 == True', {