Merge commit '60e0fc8f94210cc42fa35fcfd552851cdb6038cc' as 'third_party/abseil'

From https://github.com/abseil/abseil-cpp

Change-Id: Ie825436b781856590d9bd4a0c91ac4f41b9bef62
diff --git a/third_party/abseil/.clang-format b/third_party/abseil/.clang-format
new file mode 100644
index 0000000..06ea346
--- /dev/null
+++ b/third_party/abseil/.clang-format
@@ -0,0 +1,4 @@
+---
+Language:        Cpp
+BasedOnStyle:  Google
+...