[BackupRefPtr] Temporarily add padding

We're seeing a lot of Canary crashes that occur when BRP treats a
freelist pointer as a reference count and corrupts it. Until we find
the root cause, modify the layout of PartitionRefCount so that the
count no longer overlaps with the pointer.

Bug: 1164636
Change-Id: I39934867e042a1e54caa933e8ab82496c15f37fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2622097Reviewed-by: Bartek Nowierski <bartekn@chromium.org>
Reviewed-by: Will Harris <wfh@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Kentaro Hara <haraken@chromium.org>
Auto-Submit: Sergei Glazunov <glazunov@google.com>
Cr-Commit-Position: refs/heads/master@{#842308}

base/allocator/partition_allocator/partition_ref_count.h

@@ -85,6 +85,16 @@ class BASE_EXPORT PartitionRefCount {
   }
 
  private:
+#if defined(__clang__)
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wunused-private-field"
+#endif
+  void* padding_;  // TODO(crbug.com/1164636): This "workaround" is meant to
+                   // reduce the number of freelist corruption crashes we see in
+                   // experiments. Remove once root cause has been found.
+#if defined(__clang__)
+#pragma clang diagnostic pop
+#endif
   std::atomic<int32_t> count_{1};
 };