Disable webkit/array-iterate-backwards for debug builds.

(Debug builds have --enable-slow-asserts, and with slow asserts each
assignment to the array in the prepare function will trigger a check
over the entire array. This turns it into n^2 work, for reasonably
large n.)

BUG=

Review URL: https://codereview.chromium.org/831953003

Cr-Commit-Position: refs/heads/master@{#26006}

test/webkit/webkit.status

@@ -44,9 +44,8 @@
   'dfg-double-vote-fuzz': [SKIP],
   'reentrant-caching': [SKIP],
   'sort-large-array': [SKIP],
-  # Too slow on windows with --nocrankshaft.
-  # TODO(mstarzinger): Too slow with TF.
-  'array-iterate-backwards': [PASS, NO_VARIANTS],
+  # Too slow with --enable-slow-asserts.
+  'array-iterate-backwards': [SKIP],
 }],  # 'mode == debug'
 ['simulator', {
   # Skip tests that timeout with turbofan.
@@ -71,7 +70,6 @@
 
 ['gc_stress == True and mode == debug', {
   # Skip tests that timeout.
-  'array-iterate-backwards': [SKIP]
 }],  # 'gc_stress == True and mode == debug'
 
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