Add MinidumpSimpleStringDictionaryWriter and its test

minidump/minidump_string_writer.h

Index: minidump/minidump_string_writer.h
diff --git a/minidump/minidump_string_writer.h b/minidump/minidump_string_writer.h
index 237d0d363940067e3d68a63317653b67c60ffcff..f6c769612c5054da07197f4700998d39fcdd41ba 100644
--- a/minidump/minidump_string_writer.h
+++ b/minidump/minidump_string_writer.h
@@ -69,6 +69,11 @@ class MinidumpStringWriter : public MinidumpWritable {
   //! \note Valid in #kStateMutable.
   void set_string(const StringType& string) { string_.assign(string); }
 
+  //! \brief Retrieves the string to be written.
+  //!
+  //! \note Valid in any state.
+  const StringType& string() const { return string_; }
+
  private:
   MinidumpStringType string_base_;
   StringType string_;
@@ -113,6 +118,11 @@ class MinidumpUTF8StringWriter final
   //! \note Valid in #kStateMutable.
   void SetUTF8(const std::string& string_utf8) { set_string(string_utf8); }
 
+  //! \brief Retrieves the string to be written.
+  //!
+  //! \note Valid in any state.
+  const std::string& UTF8() const { return string(); }
+
  private:
   DISALLOW_COPY_AND_ASSIGN(MinidumpUTF8StringWriter);
 };