Small FlatMap optimization with default initialization #1535

Job	Run time
clang-compile-with-bazel	10m 10s
clang-compile-with-boringssl	11m 40s
clang-compile-with-cmake	2m 51s
clang-compile-with-bazel-all-options	11m 16s
clang-compile-with-cmake-all-options	4m 59s
clang-compile-with-make	2m 55s
clang-compile-with-make-all-options	5m 7s
clang-unittest	16m 29s
gcc-compile-with-bazel	10m 38s
gcc-compile-with-bazel-all-options	12m 46s
gcc-compile-with-boringssl	12m 25s
gcc-compile-with-cmake	3m 18s
gcc-compile-with-cmake-all-options	5m 42s
gcc-compile-with-make	2m 52s
gcc-compile-with-make-all-options	4m 59s
	1h 58m 7s

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