Add MinidumpMemoryListWriter::AddFromSnapshot(), everything downstream and its test

snapshot/mac/memory_snapshot_mac.cc

Index: snapshot/mac/memory_snapshot_mac.cc
diff --git a/snapshot/mac/memory_snapshot_mac.cc b/snapshot/mac/memory_snapshot_mac.cc
index ce6b7bcba58e6b55b1710aa5f9cd582b8e131fca..b0cce244a385366ee6b1ba3574b2fbd79e32b950 100644
--- a/snapshot/mac/memory_snapshot_mac.cc
+++ b/snapshot/mac/memory_snapshot_mac.cc
@@ -52,6 +52,11 @@ size_t MemorySnapshotMac::Size() const {
 
 bool MemorySnapshotMac::Read(Delegate* delegate) const {
   INITIALIZATION_STATE_DCHECK_VALID(initialized_);
+
+  if (size_ == 0) {
+    return delegate->MemorySnapshotDelegateRead(nullptr, size_);
+  }
+
   scoped_ptr<uint8_t[]> buffer(new uint8_t[size_]);
   if (!process_reader_->Memory()->Read(address_, size_, buffer.get())) {
     return false;