Add setIndexedDBClientCreateFunction() explanatory comment.

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

Index: third_party/WebKit/Source/modules/indexeddb/IndexedDBClient.cpp
diff --git a/third_party/WebKit/Source/modules/indexeddb/IndexedDBClient.cpp b/third_party/WebKit/Source/modules/indexeddb/IndexedDBClient.cpp
index ddbe4cd3a088f29fdb4fd10eed1cf2db736ede51..55478aab08ff6520c900dbed8931b8270fb78694 100644
--- a/third_party/WebKit/Source/modules/indexeddb/IndexedDBClient.cpp
+++ b/third_party/WebKit/Source/modules/indexeddb/IndexedDBClient.cpp
@@ -11,6 +11,10 @@ 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);