Add a new kReservedCapacity to decide when to start GC code.

Change-Id: Ib9d0ad9bc86c66ac29975f7f907f79f842342500
diff --git a/runtime/jit/jit_code_cache.cc b/runtime/jit/jit_code_cache.cc
index 74ff741..307bb6d 100644
--- a/runtime/jit/jit_code_cache.cc
+++ b/runtime/jit/jit_code_cache.cc
@@ -552,7 +552,7 @@
   // we hold the lock.
   {
     MutexLock mu(self, lock_);
-    if (!garbage_collect_code_) {
+    if (!garbage_collect_code_ || current_capacity_ < kReservedCapacity) {
       IncreaseCodeCacheCapacity();
       NotifyCollectionDone(self);
       return;