Simplify base/BUILD.gn rules for shared_memory_* files.

base/memory/shared_memory_posix.cc was especially confusing because on
iOS it was removed and then re-added to the sources list.

Change-Id: I689911144a6116e1129b2ecb5e9b90db791a7e8f
Reviewed-on: https://chromium-review.googlesource.com/794450
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520207}
1 file changed