Remove SpeechSynthesisVoice's virtual destructor
Remove SpeechSynthesisVoice's virtual destructor

Remove SpeechSynthesisVoice's destructor as it was empty and marked as virtual.
This avoids creating an unnecessary vtable as this class is never subclassed,
nor does it have any virtual function.

BUG=311561

Review URL: https://codereview.chromium.org/45053002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@160620 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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