X87: Skip slow test.

BUG=

Review-Url: https://codereview.chromium.org/2080693002
Cr-Commit-Position: refs/heads/master@{#37119}

test/webkit/webkit.status

@@ -74,6 +74,10 @@
   # Too slow.
   'dfg-int-overflow-in-loop': [SKIP],
 }],  # 'arch == s390 or arch == s390x'
+['arch == x87', {
+  # Too slow.
+  'dfg-negative-array-index': [SKIP],
+}], # 'arch == x87'
 
 ##############################################################################
 ['asan == True', {