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Currently, the RenderText classes provide access to their private/protected
members for tests using FRIEND_TEST_ALL_PREFIXES. This CL removes the need for
these macros by adding helpers to RenderTextTestApi and the test fixture
classes- RenderTextHarfBuzzTest and RenderTextMacTest.

BUG=none

Review-Url: https://codereview.chromium.org/2346733002
Cr-Commit-Position: refs/heads/master@{#419083}