Add setIndexedDBClientCreateFunction() explanatory comment.

third_party/WebKit/Source/modules/indexeddb/IndexedDBClient.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "modules/indexeddb/IndexedDBClient.h"
 #include "wtf/Atomics.h"
 
 namespace blink {
 
 static void* idbClientCreateFunction = nullptr;
 
 void setIndexedDBClientCreateFunction(CreateIndexedDBClient createFunction)
 {
+    // See web/IndexedDBClientImpl.h comment for some context. As the initialization
+    // of this IndexedDB client constructor now happens as context is set up for
+    // threads, it is possible that setIndexedDBClientCreateFunction() will be
+    // called more than once. Hence update atomicity is needed.
 #if ENABLE(ASSERT)
     CreateIndexedDBClient* currentFunction = reinterpret_cast<CreateIndexedDBClient*>(acquireLoad(&idbClientCreateFunction));
     ASSERT(!currentFunction || currentFunction == createFunction);
 #endif
     releaseStore(&idbClientCreateFunction, reinterpret_cast<void*>(createFunction));
 }
 
 IndexedDBClient* IndexedDBClient::create()
 {
     CreateIndexedDBClient* createFunction = reinterpret_cast<CreateIndexedDBClient*>(acquireLoad(&idbClientCreateFunction));
     ASSERT(createFunction);
     // There's no reason why we need to allocate a new proxy each time, but
     // there's also no strong reason not to.
     return createFunction();
 }
 
 } // namespace blink