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

Source/modules/speech/SpeechSynthesis.cpp

Index: Source/modules/speech/SpeechSynthesis.cpp
diff --git a/Source/modules/speech/SpeechSynthesis.cpp b/Source/modules/speech/SpeechSynthesis.cpp
index 00b60091d8d97ef02dbd8da2764236da39ca1907..22e026358e12f11cbff8ae0667cc6fced4ccc2ec 100644
--- a/Source/modules/speech/SpeechSynthesis.cpp
+++ b/Source/modules/speech/SpeechSynthesis.cpp
@@ -152,6 +152,7 @@ void SpeechSynthesis::fireEvent(const AtomicString& type, SpeechSynthesisUtteran
 void SpeechSynthesis::handleSpeakingCompleted(SpeechSynthesisUtterance* utterance, bool errorOccurred)
 {
     ASSERT(utterance);
+    RefPtrWillBeRawPtr<SpeechSynthesisUtterance> protect(utterance);
 
     bool didJustFinishCurrentUtterance = false;
     // If the utterance that completed was the one we're currently speaking,