Fix crash if utterance is garbage-collected before speech ends.

BUG=359130,348863

Review URL: https://codereview.chromium.org/228133002

git-svn-id: svn://svn.chromium.org/blink/trunk@171077 bbb929c8-8fbe-4397-9dbb-9b2b20218538
void SpeechSynthesis::didStartSpeaking(PassRefPtr<PlatformSpeechSynthesisUtterance> utterance)
{
    if (utterance->client())
        fireEvent(EventTypeNames::start, static_cast<SpeechSynthesisUtterance*>(utterance->client()), 0, String());
}

void SpeechSynthesis::didStartSpeaking(PassRefPtr<PlatformSpeechSynthesisUtterance> utterance)
{
    if (utterance->client())
        fireEvent(EventTypeNames::start, static_cast<SpeechSynthesisUtterance*>(utterance->client()), 0, String());
}
