Enable zlib wasm embenchen test.

gc-stress failure believed to have been fixed in:
https://crrev.com/6a806a558158cbab55ad9a1a456942a7c509c810

BUG= https://code.google.com/p/v8/issues/detail?id=4203
TEST=mjsunit/wasm/embenchen
R=aseemgarg@chromium.org,titzer@chromium.org
LOG=N

Review URL: https://codereview.chromium.org/1827813006

Cr-Commit-Position: refs/heads/master@{#35063}

test/mjsunit/mjsunit.status

@@ -280,7 +280,7 @@
   # TODO(bradnelson) Fix and re-enable.
   'wasm/embenchen/box2d': [SKIP],   # hang
   'wasm/embenchen/lua_binarytrees': [SKIP],  # fails decode
-  'wasm/embenchen/zlib': [SKIP],  # fails gc-stress
+  #'wasm/embenchen/zlib': [SKIP],  # fails gc-stress
 
   # case-insensitive unicode regexp relies on case mapping provided by ICU.
   'harmony/unicode-regexp-ignore-case': [PASS, ['no_i18n == True', FAIL]],