[9] Review Request: 6521727 DefaultTreeCellEditor doesn't implement Serializable

[Sergey Bylokhov]

Hello.
Please review a small fix for jdk9.

Bug: https://bugs.openjdk.java.net/browse/JDK-6521727
Webrev can be found at: http://cr.openjdk.java.net/~serb/6521727/webrev.00

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Best regards, Sergey.