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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}