Add FileReaderInterface::ReadExactly() and FileSeekerInterface::SeekSet()

util/file/file_writer.h

Index: util/file/file_writer.h
diff --git a/util/file/file_writer.h b/util/file/file_writer.h
index 2b0c97ed5c84715c4c6452c228c6bc83f32964aa..fd88ce08c54e99d946a45194894795e51115f488 100644
--- a/util/file/file_writer.h
+++ b/util/file/file_writer.h
@@ -42,7 +42,7 @@ struct WritableIoVec {
 
 //! \brief An interface to write to files and other file-like objects with
 //!     semantics matching the underlying platform (POSIX or Windows).
-class FileWriterInterface : public FileSeekerInterface {
+class FileWriterInterface : public virtual FileSeekerInterface {
  public:
   //! \brief Wraps LoggingWriteFile(), or provides an implementation with
   //!     identical semantics.