Ensures TTS platform is not null

This CL is moving a branch to a DCHECK.

The GetTtsPlatform() can't safely be null since other piece of code
in the same file are using the pointer without any safety check.

Example:
  if (GetTtsPlatform()->PlatformImplAvailable()) {
    GetTtsPlatform()->ClearError();
    GetTtsPlatform()->StopSpeaking();
  }

After reading the code, I came to the conclusion that it can't be
null.

The TtsPlatform::GetInstance(...) will take the tts platform
from:
  GetContentClient()->browser()->GetTtsPlatform();
If this is null, it will returns:
 TtsPlatformImpl::GetInstance() instead.

Each platform has the proper implementation PlatformImpl.

Change-Id: Ib4e88dd1be3b557dd30c3c55041e7d8d4f0e1401
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2438198
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812254}

content/browser/speech/tts_controller_impl.cc

@@ -272,13 +272,12 @@ void TtsControllerImpl::OnTtsEvent(int utterance_id,
 void TtsControllerImpl::GetVoices(BrowserContext* browser_context,
                                   std::vector<VoiceData>* out_voices) {
   TtsPlatform* tts_platform = GetTtsPlatform();
-  if (tts_platform) {
+  DCHECK(tts_platform);
   // Ensure we have all built-in voices loaded. This is a no-op if already
   // loaded.
   tts_platform->LoadBuiltInTtsEngine(browser_context);
   if (tts_platform->PlatformImplAvailable())
     tts_platform->GetVoices(out_voices);
-  }
 
   if (browser_context && engine_delegate_)
     engine_delegate_->GetVoices(browser_context, out_voices);