Merge from Chromium at DEPS revision 257591

This commit was generated by merge_to_master.py.

Change-Id: I834f3ca85c1ef7ec2c1061847a3d92aa461da043
diff --git a/Source/core/css/CSSRule.idl b/Source/core/css/CSSRule.idl
index 494e16a..a70bd3e 100644
--- a/Source/core/css/CSSRule.idl
+++ b/Source/core/css/CSSRule.idl
@@ -22,6 +22,7 @@
 [
     Custom=Wrap,
     DependentLifetime,
+    WillBeGarbageCollected,
 ] interface CSSRule {
 
     // RuleType