Add a GN arg to include PartitionAlloc sources

base/BUILD.gn

Index: base/BUILD.gn
diff --git a/base/BUILD.gn b/base/BUILD.gn
index 0c4b35fa4cec2faca98dc4d73e87fe9f4fbd18b4..a038579d95c623929ac9b6b9036ffe3c51e9fb0d 100644
--- a/base/BUILD.gn
+++ b/base/BUILD.gn
@@ -41,6 +41,9 @@ declare_args() {
   # available, except for official builds for now.
   enable_memory_task_profiler =
       use_experimental_allocator_shim && (!is_official_build || is_syzyasan)
+
+  # Partition alloc is included by default except iOS.
+  use_partition_alloc = !is_ios
 }
 
 if (is_android) {
@@ -1235,7 +1238,7 @@ component("base") {
       "synchronization/read_write_lock_nacl.cc",
     ]
 
-    if (!is_ios) {
+    if (use_partition_alloc) {
       # Add stuff that doesn't work in NaCl.
       sources += [
         # PartitionAlloc uses SpinLock, which doesn't work in NaCl (see below).
@@ -2172,7 +2175,9 @@ test("base_unittests") {
     set_sources_assignment_filter(sources_assignment_filter)
 
     # TODO(GYP): dep on copy_test_data_ios action.
-  } else {
+  }
+
+  if (use_partition_alloc) {
     sources += [ "allocator/partition_allocator/partition_alloc_unittest.cc" ]
   }