Use the UNIMPLEMENTED macro everywhere.

As suggested in review.

Change-Id: Ic14c91a622b545b4800ef021dfb501fb09cbda9d
diff --git a/src/space.cc b/src/space.cc
index 217c2ee..10d5f93 100644
--- a/src/space.cc
+++ b/src/space.cc
@@ -129,7 +129,7 @@
 }
 
 void Space::Grow(size_t new_size) {
-  LOG(FATAL) << "Unimplemented";
+  UNIMPLEMENTED(FATAL);
 }
 
 }  // namespace art