OnceCallbackHolder: Swap has_run_ to std::atomic

has_run_ is a flag used to check that a callback is only run once.
Convert from Atomic32 to std::atomic<bool> - they are broadly supported,
do not implicitly convert to/from integers, and we are converting code
to use them.

No behavior change expected.

Bug: 1194917
Change-Id: I1fca86f5cfb88e5a591f6f1af9b9dc5271ef4472
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4320832
Reviewed-by: Benoit Lize <lizeb@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: Venkatesh Srinivas <venkateshs@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1119089}

base/functional/callback_helpers.h

@@ -79,7 +79,7 @@ class OnceCallbackHolder final {
   OnceCallbackHolder& operator=(const OnceCallbackHolder&) = delete;
 
   void Run(Args... args) {
-    if (subtle::NoBarrier_AtomicExchange(&has_run_, 1)) {
+    if (has_run_.exchange(true, std::memory_order_relaxed)) {
       CHECK(ignore_extra_runs_) << "Both OnceCallbacks returned by "
                                    "base::SplitOnceCallback() were run. "
                                    "At most one of the pair should be run.";
@@ -90,7 +90,7 @@ class OnceCallbackHolder final {
   }
 
  private:
-  volatile subtle::Atomic32 has_run_ = 0;
+  std::atomic<bool> has_run_;
   base::OnceCallback<void(Args...)> callback_;
   const bool ignore_extra_runs_;
 };