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This CL is merging the TtsControllerTestHelper class into a
base testing::Test class.

Tests are converted to use that pattern:
  TEST_F(TtsControllerTest,...)

The goal of that cleanup is to simplify the next refactoring
where a content::BrowserTaskEnvironment is required for each
test.

Change-Id: Id3757b58fc05a93ac6ebacdf3ba11ec8b234e9a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2432751Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811851}