Extend PlatformSpeechSynthesizerMock with an utterance queue.

Source/modules/speech/SpeechGrammarList.cpp

Index: Source/modules/speech/SpeechGrammarList.cpp
diff --git a/Source/modules/speech/SpeechGrammarList.cpp b/Source/modules/speech/SpeechGrammarList.cpp
index 4eeefa7e3fbafb06cacb74bb01d1b5d3dc0143e9..861b534835ab3f7d5950c73d8148fe38ec0affa6 100644
--- a/Source/modules/speech/SpeechGrammarList.cpp
+++ b/Source/modules/speech/SpeechGrammarList.cpp
@@ -39,9 +39,9 @@ SpeechGrammarList* SpeechGrammarList::create()
 SpeechGrammar* SpeechGrammarList::item(unsigned index) const
 {
     if (index >= m_grammars.size())
-        return 0;
+        return nullptr;
 
-    return m_grammars[index].get();
+    return m_grammars[index];
 }
 
 void SpeechGrammarList::addFromUri(ExecutionContext* executionContext, const String& src, double weight)