Ignore bogus willSpeakWord callbacks from Mac speech synthesizer.

The Mac willSpeakWord callback occasionally returns bogus offsets,
leading to an assertion failure later. Add some code to just ignore
the callback in that case.

BUG=602776

Review-Url: https://codereview.chromium.org/2363553006
Cr-Commit-Position: refs/heads/master@{#424450}

chrome/browser/speech/tts_mac.mm

@@ -272,8 +272,9 @@ void OnSpeechEvent(NSSpeechSynthesizer* sender,
 
   if (event_type == TTS_EVENT_END)
     char_index = utterance_.size();
+
   TtsController* controller = TtsController::GetInstance();
-controller->OnTtsEvent(
+  controller->OnTtsEvent(
       utterance_id_, event_type, char_index, error_message);
   last_char_index_ = char_index;
 }
@@ -308,16 +309,26 @@ - (void)speechSynthesizer:(NSSpeechSynthesizer*)sender
 }
 
 - (void)speechSynthesizer:(NSSpeechSynthesizer*)sender
-            willSpeakWord:(NSRange)character_range
+            willSpeakWord:(NSRange)word_range
                  ofString:(NSString*)string {
+  // Ignore bogus word_range. The Mac speech synthesizer is a bit
+  // buggy and occasionally returns a number way out of range.
+  if (word_range.location > [string length])
+    return;
+
   ttsImplMac_->OnSpeechEvent(sender, TTS_EVENT_WORD,
-      character_range.location, "");
+      word_range.location, "");
 }
 
 - (void)speechSynthesizer:(NSSpeechSynthesizer*)sender
  didEncounterErrorAtIndex:(NSUInteger)character_index
                  ofString:(NSString*)string
                   message:(NSString*)message {
+  // Ignore bogus character_index. The Mac speech synthesizer is a bit
+  // buggy and occasionally returns a number way out of range.
+  if (character_index > [string length])
+    return;
+
   std::string message_utf8 = base::SysNSStringToUTF8(message);
   ttsImplMac_->OnSpeechEvent(sender, TTS_EVENT_ERROR, character_index,
       message_utf8);