Add MinidumpFileWriter::InitializeFromSnapshot() and its tests

minidump/minidump_exception_writer.cc

Index: minidump/minidump_exception_writer.cc
diff --git a/minidump/minidump_exception_writer.cc b/minidump/minidump_exception_writer.cc
index b88f46aeffcff9937ea54a1f826d880c25def59e..87382b22d366ff8442fa38f0afa7bc30ded95502 100644
--- a/minidump/minidump_exception_writer.cc
+++ b/minidump/minidump_exception_writer.cc
@@ -32,12 +32,12 @@ MinidumpExceptionWriter::~MinidumpExceptionWriter() {
 
 void MinidumpExceptionWriter::InitializeFromSnapshot(
     const ExceptionSnapshot* exception_snapshot,
-    const MinidumpThreadIDMap* thread_id_map) {
+    const MinidumpThreadIDMap& thread_id_map) {
   DCHECK_EQ(state(), kStateMutable);
   DCHECK(!context_);
 
-  auto thread_id_it = thread_id_map->find(exception_snapshot->ThreadID());
-  DCHECK(thread_id_it != thread_id_map->end());
+  auto thread_id_it = thread_id_map.find(exception_snapshot->ThreadID());
+  DCHECK(thread_id_it != thread_id_map.end());
   SetThreadID(thread_id_it->second);
 
   SetExceptionCode(exception_snapshot->Exception());