Enable DPD by default on linux debug builds

This patch aims at default enabling DPD on linux debug builds. The goal
is to provide developers instant feedback on whether their code might
trigger dangling pointers, and potentially avoid them running into the
issue on the CQ.

This first CL only enables the dangling pointers feature but DPD will
not trigger since the feature flag will still be disabled by default.

Bug: 1449923
Change-Id: Iaee0c3377c4d8078d528be75bb6499ecce3c7b6b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4574382
Commit-Queue: Paul Semel <paulsemel@chromium.org>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1164084}

build_overrides/partition_alloc.gni

@@ -80,4 +80,5 @@ enable_backup_ref_ptr_support_default = _is_brp_supported
 
 put_ref_count_in_previous_slot_default = true
 enable_backup_ref_ptr_slow_checks_default = false
-enable_dangling_raw_ptr_checks_default = false
+enable_dangling_raw_ptr_checks_default =
+    enable_backup_ref_ptr_support_default && is_linux && is_debug