[FileAPI] Check Platform availablity before using it in DOMFileSystem

Source/modules/filesystem/DOMFileSystem.cpp

 /*
  * Copyright (C) 2010 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 128 matching lines @@
     }
     OwnPtr<FileWriterCallback> m_callback;
 };
 
 }
 
 void DOMFileSystem::createWriter(const FileEntry* fileEntry, PassOwnPtr<FileWriterCallback> successCallback, PassOwnPtr<ErrorCallback> errorCallback)
 {
     ASSERT(fileEntry);
 
+    if (!fileSystem()) {
+        reportError(errorCallback, FileError::create(FileError::ABORT_ERR));
+        return;
+    }
+
     FileWriter* fileWriter = FileWriter::create(executionContext());
     OwnPtr<FileWriterBaseCallback> conversionCallback = ConvertToFileWriterCallback::create(successCallback);
     OwnPtr<AsyncFileSystemCallbacks> callbacks = FileWriterBaseCallbacks::create(fileWriter, conversionCallback.release(), errorCallback, m_context);
     fileSystem()->createFileWriter(createFileSystemURL(fileEntry), fileWriter, callbacks.release());
 }
 
 void DOMFileSystem::createFile(const FileEntry* fileEntry, PassOwnPtr<FileCallback> successCallback, PassOwnPtr<ErrorCallback> errorCallback)
 {
     KURL fileSystemURL = createFileSystemURL(fileEntry);
+    if (!fileSystem()) {
+        reportError(errorCallback, FileError::create(FileError::ABORT_ERR));
+        return;
+    }
+
     fileSystem()->createSnapshotFileAndReadMetadata(fileSystemURL, SnapshotFileCallback::create(this, fileEntry->name(), fileSystemURL, successCallback, errorCallback, m_context));
 }
 
 } // namespace blink