Add virtual destructor to EventHandler.
Add virtual destructor to EventHandler.

Change-Id: I004629c2f9853aa857ee2db8b3b6a637b801c6f7

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed