Add reminders that these classes have constexpr default constructors.

include/private/SkSpinlock.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkSpinlock_DEFINED
 #define SkSpinlock_DEFINED
 
 #include "SkTypes.h"
 #include <atomic>
 
 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)) {
             // Lock was contended.  Fall back to an out-of-line spin loop.
             this->contendedAcquire();
         }
     }
 
     void release() {
         // To act as a mutex, we need a release barrier when we release the lock.
         fLocked.store(false, std::memory_order_release);
     }
 
 private:
     SK_API void contendedAcquire();
 
     std::atomic<bool> fLocked{false};
 };
 
 #endif//SkSpinlock_DEFINED