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This cleans up the per-platform build configuration preprocessor
directives. With this change, the platform build configurations are
consolidated in one place.

Bug: 1043306
Change-Id: Id9e2da3bf156e245b510ce7b32346df285fd6f79
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2422811
Commit-Queue: Olivier Yiptong <oyiptong@chromium.org>
Auto-Submit: Olivier Yiptong <oyiptong@chromium.org>
Reviewed-by: Joshua Bell <jsbell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809240}