Small FlatMap optimization with default initialization #1533

Job	Run time
clang-compile-with-bazel	8m 51s
clang-compile-with-bazel-all-options	10m 33s
clang-compile-with-cmake	2m 46s
clang-compile-with-boringssl	9m 5s
clang-compile-with-cmake-all-options	4m 50s
clang-compile-with-make	2m 56s
clang-compile-with-make-all-options	5m 15s
clang-unittest	11m 37s
gcc-compile-with-bazel	9m 26s
gcc-compile-with-bazel-all-options	9m 31s
gcc-compile-with-boringssl	7m 30s
gcc-compile-with-cmake	3m 23s
gcc-compile-with-cmake-all-options	5m 30s
gcc-compile-with-make	3m 19s
gcc-compile-with-make-all-options	5m 9s
	1h 39m 41s

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