[PA] Improve default for ASAN BRP arg

ASAN BRP is not supported in standalone PA (i.e. in external embedders).
This CL ensures that `partition_alloc.gni` doesn't attempt to enable it
even when the invoker passes `is_asan = true`.

Change-Id: I1e82f8a379484dffbade3f0addee657741ccb294
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4084284
Reviewed-by: Takashi Sakamoto <tasak@google.com>
Commit-Queue: Kalvin Lee <kdlee@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1080167}

base/allocator/partition_allocator/partition_alloc.gni

@@ -80,8 +80,9 @@ assert(!enable_pointer_compression_support || glue_core_pools,
 declare_args() {
   # The supported platforms are supposed to match `_is_brp_supported`, but we
   # enable the feature on Linux early because it's most widely used for security
-  # research
-  use_asan_backup_ref_ptr = is_asan && (is_win || is_android || is_linux)
+  # research. ASAN BRP is not supported in Chromium-external builds.
+  use_asan_backup_ref_ptr =
+      build_with_chromium && is_asan && (is_win || is_android || is_linux)
 }
 
 # AsanBackupRefPtr is not supported outside Chromium. The implementation is