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Consolidate some build rules in base/BUILD.gn by OS.

Change-Id: Ib785f3bab568f15a4d970e185cd6c6eedd6e806d
Reviewed-on: https://chromium-review.googlesource.com/1196236
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587939}