Add reminders that these classes have constexpr default constructors.

include/private/SkSpinlock.h

Index: include/private/SkSpinlock.h
diff --git a/include/private/SkSpinlock.h b/include/private/SkSpinlock.h
index 27442068717b319f6bd56cda1fc327150b71f057..a5d378289ffa1eb47dbd142b07df58f491c6f136 100644
--- a/include/private/SkSpinlock.h
+++ b/include/private/SkSpinlock.h
@@ -13,6 +13,8 @@
 
 class SkSpinlock {
 public:
+    constexpr SkSpinlock() = default;
+
     void acquire() {
         // To act as a mutex, we need an acquire barrier when we acquire the lock.
         if (fLocked.exchange(true, std::memory_order_acquire)) {