Extend PlatformSpeechSynthesizerMock with an utterance queue.

LayoutTests/fast/speechsynthesis/speech-synthesis-speak-multiple-expected.txt

Index: LayoutTests/fast/speechsynthesis/speech-synthesis-speak-multiple-expected.txt
diff --git a/LayoutTests/fast/speechsynthesis/speech-synthesis-speak-multiple-expected.txt b/LayoutTests/fast/speechsynthesis/speech-synthesis-speak-multiple-expected.txt
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+++ b/LayoutTests/fast/speechsynthesis/speech-synthesis-speak-multiple-expected.txt
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+This tests that multiple speak utterances are handled in sequence.
+
+On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
+
+
+PASS speechSynthesis.speaking is false
+PASS event.elapsedTime > 0 is true
+PASS speechSynthesis.speaking is true
+PASS count is >= i
+PASS event.elapsedTime > 0 is true
+PASS speechSynthesis.speaking is true
+PASS event.elapsedTime > 0 is true
+PASS speechSynthesis.speaking is true
+PASS count is >= i
+PASS event.elapsedTime > 0 is true
+PASS speechSynthesis.speaking is true
+PASS event.elapsedTime > 0 is true
+PASS speechSynthesis.speaking is true
+PASS count is >= i
+PASS event.elapsedTime > 0 is true
+PASS speechSynthesis.speaking is true
+PASS successfullyParsed is true
+
+TEST COMPLETE
+