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/heap.cc b/src/heap.cc
index 1303d7d..7261670 100644
--- a/src/heap.cc
+++ b/src/heap.cc
@@ -914,8 +914,11 @@
 }
 
 void Heap::RequestConcurrentGC() {
-  // Make sure that our Daemon threads are started
-  if (requesting_gc_ || !Runtime::Current()->IsFinishedStarting() || Runtime::Current()->IsShuttingDown()) {
+  // Make sure that we can do a concurrent GC.
+  if (requesting_gc_ ||
+      !Runtime::Current()->IsFinishedStarting() ||
+      Runtime::Current()->IsShuttingDown() ||
+      !Runtime::Current()->IsConcurrentGcEnabled()) {
     return;
   }