Enable / disable CMS runtime options.

Enable or disable CMS by doing -Xgc:noconcurrent or Xgc:concurrent. Defaults to enabled.

Change-Id: Ib18687ba6aa6534323531d761133ce410c69f426
diff --git a/src/runtime.h b/src/runtime.h
index 9181f1f..698f0c5 100644
--- a/src/runtime.h
+++ b/src/runtime.h
@@ -70,6 +70,7 @@
     std::string jni_trace_;
     bool is_compiler_;
     bool is_zygote_;
+    bool is_concurrent_gc_enabled_;
     size_t heap_initial_size_;
     size_t heap_maximum_size_;
     size_t heap_growth_limit_;
@@ -101,6 +102,10 @@
     return is_zygote_;
   }
 
+  bool IsConcurrentGcEnabled() const {
+    return is_concurrent_gc_enabled_;
+  }
+
   const std::string& GetHostPrefix() const {
     DCHECK(!IsStarted());
     return host_prefix_;
@@ -329,6 +334,7 @@
 
   bool is_compiler_;
   bool is_zygote_;
+  bool is_concurrent_gc_enabled_;
 
   // The host prefix is used during cross compilation. It is removed
   // from the start of host paths such as: