Use std::atomic_ulong for CreateUniqueIdentifier()

We will use CreateUniqueIdentifier() from worker threads when off-main-thread-fetch is enabled.
So g_unique_identifier must be atomic.

Bug: 443374
Change-Id: I766f466c250a90a8e2e61c8ac5535d3144a909b6
Reviewed-on: https://chromium-review.googlesource.com/593467
Commit-Queue: Tsuyoshi Horo <horo@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490829}

third_party/WebKit/Source/platform/loader/fetch/UniqueIdentifier.cpp

@@ -28,14 +28,16 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#include <atomic>
+
 #include "platform/loader/fetch/UniqueIdentifier.h"
 
 namespace blink {
 
-static unsigned long g_unique_identifier = 0;
+static std::atomic_ulong g_unique_identifier(1);
 
 unsigned long CreateUniqueIdentifier() {
-  return ++g_unique_identifier;
+  return g_unique_identifier.fetch_add(1, std::memory_order_relaxed);
 }
 
 }  // namespace blink