Suppress leak in Blink on mac valgrind bot

tools/valgrind/memcheck/suppressions_mac.txt

 # There are three kinds of suppressions in this file:
 # 1. Third party stuff we have no control over.
 #
 # 2. Intentional unit test errors, stuff that is somehow a false positive
 # in our own code, or stuff that is so trivial it's not worth fixing.
 #
 # 3. Suppressions for real chromium bugs that are not yet fixed.
 # These should all be in chromium's bug tracking system.
 # Periodically we should sweep this file and the bug tracker clean by
 # running overnight and removing outdated bugs/suppressions.
@@ skipping 2329 matching lines @@
    fun:_ZN5blink11WebBindings*get*Identifier*
 }
 {
    bug_310761_b
    Memcheck:Leak
    fun:malloc
    fun:_ZN3WTF10fastMallocEm
    fun:_ZN7WebCore*TerminatedArrayBuilder*
    fun:_ZN7WebCore7RuleSet19compactPendingRules*
 }
+{
+   bug_320918
+   Memcheck:Leak
+   fun:malloc
+   fun:_ZN3WTF10fastMallocEm
+   fun:_ZN3WTF11LinkedStackIN7WebCore8RuleDataEEnwEm
+   fun:_ZN7WebCore7RuleSet12addToRuleSet*
+   fun:_ZN7WebCore7RuleSet21findBestRuleSetAndAdd*
+   fun:_ZN7WebCore7RuleSet7addRule*
+   fun:_ZN7WebCore7RuleSet13addChildRules*
+   fun:_ZN7WebCore7RuleSet17addRulesFromSheet*
+   fun:_ZN7WebCore21CSSDefaultStyleSheets16loadDefaultStyle*
+}