Add MinidumpMemoryWriter, MinidumpMemoryListWriter, and their test

minidump/minidump_writable_test.cc

Index: minidump/minidump_writable_test.cc
diff --git a/minidump/minidump_writable_test.cc b/minidump/minidump_writable_test.cc
index 616f8cd8c716d2414101801710856893f13659cc..fb52334dbd9389327ce329665c2cab20b9e03e0b 100644
--- a/minidump/minidump_writable_test.cc
+++ b/minidump/minidump_writable_test.cc
@@ -133,8 +133,9 @@ class TestStringMinidumpWritable final : public BaseTestMinidumpWritable {
 
   virtual bool WriteObject(FileWriterInterface* file_writer) override {
     BaseTestMinidumpWritable::WriteObject(file_writer);
-    EXPECT_TRUE(file_writer->Write(&data_[0], data_.size()));
-    return true;
+    bool rv = file_writer->Write(&data_[0], data_.size());
+    EXPECT_TRUE(rv);
+    return rv;
   }
 
  private: