Add ProcessSnapshotMinidump, the beginning of the minidump variant of the Snapshot family

snapshot/mac/process_snapshot_mac.cc

Index: snapshot/mac/process_snapshot_mac.cc
diff --git a/snapshot/mac/process_snapshot_mac.cc b/snapshot/mac/process_snapshot_mac.cc
index dd462a9020beed7c63dbbeb1b2fceb218b38b717..767c75a049fac3a7bd91bb9d0c1e2ff571fe88b6 100644
--- a/snapshot/mac/process_snapshot_mac.cc
+++ b/snapshot/mac/process_snapshot_mac.cc
@@ -107,6 +107,7 @@ void ProcessSnapshotMac::ProcessCPUTimes(timeval* user_time,
 
 const std::map<std::string, std::string>&
 ProcessSnapshotMac::AnnotationsSimpleMap() const {
+  INITIALIZATION_STATE_DCHECK_VALID(initialized_);
   return annotations_simple_map_;
 }