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Since use_allocator="partition" means that PartitionAlloc should
be used as the default allocator, select the partition alloc shim
in case of use_allocator="partition".

This patch doesn't take effect immediately as no bots currently
specify use_allocator="partition" (PA-Everywhere is WIP).

When PA-Everywhere will land, it will be enabled on all eligible
platforms (Linux, Mac, Win, Android, CrOS, but not iOS), i.e.
PA-Everywhere will be enabled on Linux as target and host env,
Android as target env, and CrOS as target env, so there is no
need to support a twisted case like PA-E on Android (target) +
glibc on Linux (host).  This patch simplifies and fixes the build
settings.

Bug: 998048
Change-Id: I7ae736e58e9b77185f7ad046f8cc5df56d9c02ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2362352
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Benoit L <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805257}