scoped_nsobject, scoped_nsprotocol and scoped_block do not currently support ARC.

Making these smart pointers to support both MRC and ARC is the 1st step to start the ARC migration for the embedder.

As pointed out by @stuartmorgan in flutter/engine#18362 (comment), https://codereview.chromium.org/1855483004 is the commit that Chromium made them work with both MRC and ARC. Since the implementation of these smart pointers were originally adopted from Chromium's implementation, we should be able to adopt their strategy again without much of change.