Make it possible to enable tlab with a build parameter.

For testing.

Change-Id: I4d06a4db733b7fa09105f068028483cf88db7925
diff --git a/runtime/globals.h b/runtime/globals.h
index 0845475..ac8751c 100644
--- a/runtime/globals.h
+++ b/runtime/globals.h
@@ -109,6 +109,13 @@
 static constexpr bool kPoisonHeapReferences = false;
 #endif
 
+// If true, enable the tlab allocator by default.
+#ifdef ART_USE_TLAB
+static constexpr bool kUseTlab = true;
+#else
+static constexpr bool kUseTlab = false;
+#endif
+
 // Kinds of tracing clocks.
 enum class TraceClockSource {
   kThreadCpu,
diff --git a/runtime/runtime_options.def b/runtime/runtime_options.def
index 8775f8d..895ab5c 100644
--- a/runtime/runtime_options.def
+++ b/runtime/runtime_options.def
@@ -62,7 +62,7 @@
 RUNTIME_OPTIONS_KEY (Unit,                DumpJITInfoOnShutdown)
 RUNTIME_OPTIONS_KEY (Unit,                IgnoreMaxFootprint)
 RUNTIME_OPTIONS_KEY (Unit,                LowMemoryMode)
-RUNTIME_OPTIONS_KEY (bool,                UseTLAB,                        false)
+RUNTIME_OPTIONS_KEY (bool,                UseTLAB,                        kUseTlab)
 RUNTIME_OPTIONS_KEY (bool,                EnableHSpaceCompactForOOM,      true)
 RUNTIME_OPTIONS_KEY (bool,                UseJIT,      false)
 RUNTIME_OPTIONS_KEY (unsigned int,        JITCompileThreshold, jit::Jit::kDefaultCompileThreshold)