Add external byte array API and finalize external strings and byte arrays.

runtime/vm/raw_object_snapshot.cc

Index: runtime/vm/raw_object_snapshot.cc
diff --git a/runtime/vm/raw_object_snapshot.cc b/runtime/vm/raw_object_snapshot.cc
index 7b515c5742bb93746fc763cf0a723c13f9979ee9..437f4f8c6ff2c9466d479d6d5c3d1d8febf6dec5 100644
--- a/runtime/vm/raw_object_snapshot.cc
+++ b/runtime/vm/raw_object_snapshot.cc
@@ -1479,7 +1479,7 @@ void RawExternalOneByteString::WriteTo(SnapshotWriter* writer,
                 ptr()->tags_,
                 ptr()->length_,
                 ptr()->hash_,
-                ptr()->external_data_->data_);
+                ptr()->external_data_->data());
 }
 
 
@@ -1494,7 +1494,7 @@ void RawExternalTwoByteString::WriteTo(SnapshotWriter* writer,
                 ptr()->tags_,
                 ptr()->length_,
                 ptr()->hash_,
-                ptr()->external_data_->data_);
+                ptr()->external_data_->data());
 }
 
 
@@ -1509,7 +1509,7 @@ void RawExternalFourByteString::WriteTo(SnapshotWriter* writer,
                 ptr()->tags_,
                 ptr()->length_,
                 ptr()->hash_,
-                ptr()->external_data_->data_);
+                ptr()->external_data_->data());
 }
 
 
@@ -1738,7 +1738,7 @@ void RawExternalByteArray::WriteTo(SnapshotWriter* writer,
                    ObjectStore::kInternalByteArrayClass,
                    ptr()->tags_,
                    ptr()->length_,
-                   ptr()->data_);
+                   ptr()->external_data_->data());
 }