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
bool SpeechSynthesis::pending() const
{
    return m_utteranceQueue.size() > 1;
}

bool SpeechSynthesis::pending() const
{
    return m_utteranceQueue.size() > 1;
}
