Add -xGc: MS, CMS, SS options to specify which GC to use.

Can be used for running tests or benchmarks with semispace,
marksweep or concurrent marksweep.

Change-Id: Ic9ab1220150f2c7c9c30df4ffee45b9d303094b3
diff --git a/runtime/runtime.h b/runtime/runtime.h
index 0140ddb..e6951d9 100644
--- a/runtime/runtime.h
+++ b/runtime/runtime.h
@@ -27,6 +27,7 @@
 
 #include "base/macros.h"
 #include "base/stringpiece.h"
+#include "gc/collector_type.h"
 #include "gc/heap.h"
 #include "globals.h"
 #include "instruction_set.h"
@@ -99,7 +100,6 @@
     bool is_compiler_;
     bool is_zygote_;
     bool interpreter_only_;
-    bool is_concurrent_gc_enabled_;
     bool is_explicit_gc_disabled_;
     size_t long_pause_log_threshold_;
     size_t long_gc_log_threshold_;
@@ -113,6 +113,7 @@
     double heap_target_utilization_;
     size_t parallel_gc_threads_;
     size_t conc_gc_threads_;
+    gc::CollectorType collector_type_;
     size_t stack_size_;
     size_t max_spins_before_thin_lock_inflation_;
     bool low_memory_mode_;