Upgrade to Google Benchmark 1.6.0

Change-Id: I0e5993ca4352d34f08e6a22e392340d4b68f390a
Reviewed-on: http://review.couchbase.org/c/platform/+/161690
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>

tests/arena_malloc/arena_tracking_bench.cc

@@ -42,7 +42,7 @@ class MemoryAllocationStat : public benchmark::Fixture {
 };
 
 BENCHMARK_DEFINE_F(MemoryAllocationStat, AllocNRead1)(benchmark::State& state) {
-    if (state.thread_index == 0) {
+    if (state.thread_index() == 0) {
         // memUsed merge must be 4 times higher so in theory we merge at the
         // same rate as TLS (because 4 more threads than cores).
         client.estimateUpdateThreshold = 10240 * 4;
@@ -64,7 +64,7 @@ BENCHMARK_DEFINE_F(MemoryAllocationStat, AllocNRead1)(benchmark::State& state) {
 }
 
 BENCHMARK_DEFINE_F(MemoryAllocationStat, AllocNReadM)(benchmark::State& state) {
-    if (state.thread_index == 0) {
+    if (state.thread_index() == 0) {
         // memUsed merge must be 4 times higher so in theory we merge at the
         // same rate as TLS (because 4 more threads than cores).
         client.estimateUpdateThreshold = 10240 * 4;
@@ -88,7 +88,7 @@ BENCHMARK_DEFINE_F(MemoryAllocationStat, AllocNReadM)(benchmark::State& state) {
 
 BENCHMARK_DEFINE_F(MemoryAllocationStat, AllocNReadPreciseM)
 (benchmark::State& state) {
-    if (state.thread_index == 0) {
+    if (state.thread_index() == 0) {
         // memUsed merge must be 4 times higher so in theory we merge at the
         // same rate as TLS (because 4 more threads than cores).
         client.estimateUpdateThreshold = 10240 * 4;