Skip slow webkit/array-iterate-backwards in arm64.debug/gc-stress mode.

BUG=
TBR=hpayer@chromium.org

Review URL: https://codereview.chromium.org/795633002

Cr-Commit-Position: refs/heads/master@{#25755}

test/webkit/webkit.status

@@ -70,8 +70,8 @@
 }],  # 'gc_stress == True'
 
 ['gc_stress == True and mode == debug', {
-  # Skip tests that timout with turbofan.
-  'array-iterate-backwards': [PASS, NO_VARIANTS]
+  # Skip tests that timeout.
+  'array-iterate-backwards': [SKIP]
 }],  # 'gc_stress == True and mode == debug'
 
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