Small FlatMap optimization with default initialization #1536

Job	Run time
clang-compile-with-bazel	10m 14s
clang-compile-with-bazel-all-options	11m 26s
clang-compile-with-boringssl	12m 2s
clang-compile-with-cmake	48s
clang-compile-with-cmake-all-options	3m 11s
clang-compile-with-make	1m 7s
clang-compile-with-make-all-options	2m 56s
clang-unittest	1m 22s
gcc-compile-with-bazel	10m 35s
gcc-compile-with-cmake	46s
gcc-compile-with-boringssl	12m 26s
gcc-compile-with-bazel-all-options	13m 20s
gcc-compile-with-cmake-all-options	3m 14s
gcc-compile-with-make	59s
gcc-compile-with-make-all-options	3m 4s
	1h 27m 30s

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