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This replaces the current WIDEVINE_CDM_IS_COMPONENT macro defined in
widevine_cdm_common.h. Buildflag is safer, also this avoids tricky cases
where widevine_cdm_common.h must be included before the define can be
evaluated.

Bug: 349182
Test: No functionality change
Change-Id: Ifc26d2fa87b00997e9335a977cb6417c4ec4432d
Reviewed-on: https://chromium-review.googlesource.com/1248132Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: John Rummell <jrummell@chromium.org>
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594961}