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Currently, on Mac most of the tests in render_text_unittest.cc use
RenderText::CreateInstance to create their render text instance. Also, many
tests which would fail on Mac with RenderTextMac are not compiled on Mac. As a
result there is negligible test coverage for RenderTextHarfBuzz on Mac.

This CL uses parameterized tests to test both RenderTextMac and
RenderTextHarfBuzz on Mac. It does this by introducing three new test fixture
classes which support parameterized tests-
-RenderTextTest
-RenderTextHarfBuzzTest
-RenderTextMacTest
The test helper class RenderTextAllBackends is removed.

Before this CL, 65 tests in render_text_unittest.cc are run on Mac. After this
CL, a total of 129 tests in render_text_unittest.cc are run on Mac.

BUG=639194

Review-Url: https://codereview.chromium.org/2251893004
Cr-Commit-Position: refs/heads/master@{#414644}