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This #define has been error-prone in the past and now no longer matches
the Widevine convention after which it was modeled. Convert to use the
much-safer BUILDFLAG() macro. Update use_playready to enable_playready
to match the Chromium GN conventions.

BUG=

Test: Cast CQ
Change-Id: I5ac8a238051a6208d2960ee7ce957099b396e45a
Reviewed-on: https://chromium-review.googlesource.com/c/1284065Reviewed-by: Yuchen Liu <yucliu@chromium.org>
Commit-Queue: Stephen Lanham <slan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600154}