Merge changes from upstream

Commands:
git clone https://fuchsia.googlesource.com/third_party/abseil-cpp
cd abseil-cpp
git checkout upstream/master
git checkout origin/master -b ${USER}-merge
git merge upstream/master

New version of Tink needed by Cobalt uses flat_hash_map.

Change-Id: Icefc708c0b6dedf3ef4981f7b19ca2036a3c9a6b