Add FileReaderInterface. Move StringFileWriter to StringFile and

util/file/file_seeker.h

Index: util/file/file_seeker.h
diff --git a/util/file/file_seeker.h b/util/file/file_seeker.h
new file mode 100644
index 0000000000000000000000000000000000000000..f6d8b46461f0bbbd0c2fcb9ae2bfbf1e544f7cd9
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+++ b/util/file/file_seeker.h
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+// Copyright 2015 The Crashpad Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef CRASHPAD_UTIL_FILE_FILE_SEEKER_H_
+#define CRASHPAD_UTIL_FILE_FILE_SEEKER_H_
+
+#include "util/file/file_io.h"
+
+namespace crashpad {
+
+//! \brief An interface to seek in files and other file-like objects with
+//!     semantics matching the underlying platform (POSIX or Windows).
+class FileSeekerInterface {
+ public:
+  //! \brief Wraps LoggingFileSeek() or provides an alternate implementation
+  //!     with identical semantics.
+  //!
+  //! \return The return value of LoggingFileSeek(). `-1` on failure,
+  //!     with an error message logged.
+  virtual FileOffset Seek(FileOffset offset, int whence) = 0;
+
+ protected:
+  ~FileSeekerInterface() {}
+};
+
+}  // namespace crashpad
+
+#endif  // CRASHPAD_UTIL_FILE_FILE_SEEKER_H_