Small FlatMap optimization with default initialization #1528

Job	Run time
clang-compile-with-bazel	9m 27s
clang-compile-with-boringssl	12m 42s
clang-compile-with-cmake	3m 46s
clang-compile-with-bazel-all-options	10m 55s
clang-compile-with-cmake-all-options	5m 1s
clang-compile-with-make	3m 1s
clang-compile-with-make-all-options	4m 55s
clang-unittest	12m 26s
gcc-compile-with-bazel	10m 27s
gcc-compile-with-bazel-all-options	12m 38s
gcc-compile-with-cmake	55s
gcc-compile-with-boringssl	12m 37s
gcc-compile-with-cmake-all-options	4m 18s
gcc-compile-with-make	1m 0s
gcc-compile-with-make-all-options	2m 48s
	1h 46m 56s

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