Add MinidumpMemoryWriter, MinidumpMemoryListWriter, and their test

minidump/minidump_writable.h

Index: minidump/minidump_writable.h
diff --git a/minidump/minidump_writable.h b/minidump/minidump_writable.h
index 32ed81e714d3c73f7c06506fd138af2f12b87ed9..15876d4896acede4bb6210620d9be13e2a21612a 100644
--- a/minidump/minidump_writable.h
+++ b/minidump/minidump_writable.h
@@ -259,8 +259,11 @@ class MinidumpWritable {
   virtual bool WriteObject(FileWriterInterface* file_writer) = 0;
 
  private:
-  std::vector<RVA*> registered_rvas_;
+  std::vector<RVA*> registered_rvas_;  // weak
+
+  // weak
   std::vector<MINIDUMP_LOCATION_DESCRIPTOR*> registered_location_descriptors_;
+
   size_t leading_pad_bytes_;
   State state_;