Make UseTLAB a bool flag.

This would make it easier to override its default value at build time.

Change-Id: I643fe1afaf492f0443dc01df0c9a62bb3fa025d9
diff --git a/runtime/runtime.cc b/runtime/runtime.cc
index c6e858b..0ccc97c 100644
--- a/runtime/runtime.cc
+++ b/runtime/runtime.cc
@@ -787,7 +787,7 @@
                        runtime_options.GetOrDefault(Opt::LongPauseLogThreshold),
                        runtime_options.GetOrDefault(Opt::LongGCLogThreshold),
                        runtime_options.Exists(Opt::IgnoreMaxFootprint),
-                       runtime_options.Exists(Opt::UseTLAB),
+                       runtime_options.GetOrDefault(Opt::UseTLAB),
                        xgc_option.verify_pre_gc_heap_,
                        xgc_option.verify_pre_sweeping_heap_,
                        xgc_option.verify_post_gc_heap_,