Don't create a TTS platform impl if none exists on VoicesChanged.

This fixes a regression where the platform impl was created even
when it wasn't yet needed.

Bug: 911184
Change-Id: I6555a81ff86327fd12994bf4de5e44074dfeee15
Reviewed-on: https://chromium-review.googlesource.com/c/1358919Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Katie Dektar <katie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613207}

chrome/browser/speech/tts_controller_delegate_impl.cc

@@ -546,7 +546,7 @@ const PrefService* TtsControllerDelegateImpl::GetPrefService(
 void TtsControllerDelegateImpl::VoicesChanged() {
   // Existence of platform tts indicates explicit requests to tts. Since
   // |VoicesChanged| can occur implicitly, only send if needed.
-  if (!GetTtsPlatform())
+  if (!tts_platform_)
     return;
 
   for (auto& delegate : voices_changed_delegates_)