IndexedDB: Simplify WebIDBCallbacks exports, strip IDBFactoryBackend

Source/modules/modules.gypi

 {
   'variables': {
     # Experimental hooks for embedder to provide extra IDL and source files.
     #
     # Note: this is not a supported API. If you rely on this, you will be broken
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     'extra_blink_module_idl_files': [],
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     'modules_idl_files': [
@@ skipping 375 matching lines @@
       'indexeddb/IDBRequest.cpp',
       'indexeddb/IDBRequest.h',
       'indexeddb/IDBTracing.h',
       'indexeddb/IDBTransaction.cpp',
       'indexeddb/IDBTransaction.h',
       'indexeddb/IDBVersionChangeEvent.cpp',
       'indexeddb/IDBVersionChangeEvent.h',
       'indexeddb/IndexedDB.h',
       'indexeddb/PageGroupIndexedDatabase.cpp',
       'indexeddb/PageGroupIndexedDatabase.h',
+      'indexeddb/WebIDBCallbacksImpl.cpp',
+      'indexeddb/WebIDBCallbacksImpl.h',
       'indexeddb/WebIDBDatabaseCallbacksImpl.cpp',
       'indexeddb/WebIDBDatabaseCallbacksImpl.h',
       'indexeddb/WorkerGlobalScopeIndexedDatabase.cpp',
       'indexeddb/WorkerGlobalScopeIndexedDatabase.h',
       'indexeddb/chromium/IDBFactoryBackendInterfaceChromium.cpp',
       'indexeddb/chromium/IDBFactoryBackendInterfaceChromium.h',
       'mediasource/HTMLVideoElementMediaSource.cpp',
       'mediasource/HTMLVideoElementMediaSource.h',
       'mediasource/MediaSource.cpp',
       'mediasource/MediaSource.h',
@@ skipping 387 matching lines @@
       'indexeddb/IDBKeyPathTest.cpp',
       'indexeddb/IDBRequestTest.cpp',
       'indexeddb/IDBTransactionTest.cpp',
       'websockets/WebSocketDeflaterTest.cpp',
       'websockets/WebSocketExtensionDispatcherTest.cpp',
       'websockets/WebSocketExtensionParserTest.cpp',
       'websockets/WebSocketPerMessageDeflateTest.cpp',
     ],
   },
 }