Vector Downward GetSize optimization (#6925)

* Added Google benchmarks (and gtests)

* Separate benchmark CMakeLists.txt to its own file

* Move output directory to target just flatbenchmark

* Reduced from encoding 210ns -> 188ns

* store size_ as uoffset_t

* fixed windows c4267 warning

benchmarks/CMakeLists.txt

@@ -37,6 +37,7 @@ set(FlatBenchmark_SRCS
     ${CPP_BENCH_DIR}/benchmark_main.cpp
     ${CPP_FB_BENCH_DIR}/fb_bench.cpp
     ${CPP_RAW_BENCH_DIR}/raw_bench.cpp
+    ${CPP_BENCH_FB_GEN}
 )
 
 # Generate the flatbuffers benchmark code from the flatbuffers schema using