Fix Fuchsia compile bug with TtsPlatform.

Bug: 910653
Change-Id: Id6009d4789c560e54c450377c16f1013e57a914c
Reviewed-on: https://chromium-review.googlesource.com/c/1356886Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Katie Dektar <katie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612744}

content/browser/speech/tts_platform_impl.cc

@@ -14,7 +14,11 @@ namespace content {
 // static
 TtsPlatform* TtsPlatform::GetInstance() {
 #if defined(OS_CHROMEOS) || defined(OS_ANDROID)
+  // Chrome and Android TTS platforms have chrome/ dependencies.
   return GetContentClient()->browser()->GetTtsPlatform();
+#elif defined(OS_FUCHSIA)
+  // There is no platform TTS definition for Fuchsia.
+  return nullptr;
 #else
   return TtsPlatformImpl::GetInstance();
 #endif