initialize LogStream._backtrace to fixed randomly printed stacktraces #1564

Job	Run time
clang-compile-with-bazel	15m 56s
clang-compile-with-bazel-all-options	17m 40s
clang-compile-with-boringssl	16m 53s
clang-compile-with-cmake	2m 58s
clang-compile-with-cmake-all-options	5m 8s
clang-compile-with-make	3m 0s
clang-compile-with-make-all-options	5m 17s
gcc-compile-with-bazel	16m 52s
gcc-compile-with-bazel-all-options	17m 57s
clang-unittest	15m 53s
gcc-compile-with-boringssl	17m 51s
gcc-compile-with-cmake	3m 58s
gcc-compile-with-cmake-all-options	5m 40s
gcc-compile-with-make	3m 1s
gcc-compile-with-make-all-options	5m 23s
	2h 33m 27s

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