RFR: 8253606: Need to add missed constructor to the SwingEventMonitor

[Sergey Bylokhov]

The javadoc for this class was added, but the constructor itsef is missed......

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Commit messages:
 - Update SwingEventMonitor.java

Changes: https://git.openjdk.java.net/jdk/pull/340/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=340&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253606
  Stats: 9 lines in 1 file changed: 5 ins; 4 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/340.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/340/head:pull/340

PR: https://git.openjdk.java.net/jdk/pull/340