WebViewTestProxyBase: clear out main test delegate upon destruction.

content/shell/test_runner/mock_web_speech_recognizer.cc

Index: content/shell/test_runner/mock_web_speech_recognizer.cc
diff --git a/content/shell/test_runner/mock_web_speech_recognizer.cc b/content/shell/test_runner/mock_web_speech_recognizer.cc
index 56c35ae8afa6561d29378c08f317ba4fa7b4816b..29ca528b32e791bc4d58dcf43bc5fc2c5e2ae1bd 100644
--- a/content/shell/test_runner/mock_web_speech_recognizer.cc
+++ b/content/shell/test_runner/mock_web_speech_recognizer.cc
@@ -160,7 +160,7 @@ MockWebSpeechRecognizer::MockWebSpeechRecognizer()
       weak_factory_(this) {}
 
 MockWebSpeechRecognizer::~MockWebSpeechRecognizer() {
-  ClearTaskQueue();
+  SetDelegate(nullptr);
 }
 
 bool MockWebSpeechRecognizer::Task::isNewContextTask() const {
@@ -169,6 +169,9 @@ bool MockWebSpeechRecognizer::Task::isNewContextTask() const {
 
 void MockWebSpeechRecognizer::SetDelegate(WebTestDelegate* delegate) {
   delegate_ = delegate;
+  // No delegate to forward to, clear out pending tasks.
+  if (!delegate_)
+    ClearTaskQueue();
 }
 
 void MockWebSpeechRecognizer::SetClientContext(