base/allocator: Fix android_cronet build with PA-Everywhere

https://crrev.com/c/2300380 is failing to build on android_cronet
bot due to a conflict in GN settings between
use_allocator=="partition" and use_partition_alloc=false.

Ideally, we shouldn't have two flags and we'd better to remove
use_partition_alloc in future.  This patch mitigates the issue
by prioritizing use_allocator over use_partition_alloc.

Bug: 998048
Change-Id: I293a884186dfa9804930b835ba236efb818a761d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2368797
Reviewed-by: Benoit L <lizeb@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801737}
GitOrigin-RevId: 24622fa5673f6a05341cb5b6841dac331f3bfcb0

config/allocator.gni

@@ -1,10 +0,0 @@
-# Copyright 2014 The Chromium Authors. All rights reserved.
-# Use of this source code is governed by a BSD-style license that can be
-# found in the LICENSE file.
-
-declare_args() {
-  # Partition alloc is included by default except iOS.
-  # TODO(thakis): Move this elsewhere, probably
-  # base/allocator/partition_allocator/buildflags.gni
-  use_partition_alloc = !is_ios
-}