Switch [String]FileWriter to use new file_io.h functions/types

util/file/string_file_writer.h

Index: util/file/string_file_writer.h
diff --git a/util/file/string_file_writer.h b/util/file/string_file_writer.h
index fa1e7de938cac21dac319189515f7f47d6e7393d..bdfb5e55902a1e31bf42edf4f5b329f57bbd9eea 100644
--- a/util/file/string_file_writer.h
+++ b/util/file/string_file_writer.h
@@ -47,7 +47,7 @@ class StringFileWriter : public FileWriterInterface {
   // FileWriterInterface:
   bool Write(const void* data, size_t size) override;
   bool WriteIoVec(std::vector<WritableIoVec>* iovecs) override;
-  off_t Seek(off_t offset, int whence) override;
+  FileOffset Seek(FileOffset offset, int whence) override;
 
  private:
   //! \brief The virtual file’s contents.