Add support for a target new space size

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index 842a2c13b2714965508f84a2908826fd3824a46e..9baf92c50331aeb267203d13df91be46e0c0cbbd 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -566,6 +566,7 @@ class Heap {
   int MaxSemiSpaceSize() { return max_semi_space_size_; }
   int ReservedSemiSpaceSize() { return reserved_semispace_size_; }
   int InitialSemiSpaceSize() { return initial_semispace_size_; }
+  int TargetSemiSpaceSize() { return target_semispace_size_; }
   intptr_t MaxOldGenerationSize() { return max_old_generation_size_; }
   intptr_t MaxExecutableSize() { return max_executable_size_; }
 
@@ -1468,6 +1469,7 @@ class Heap {
   int reserved_semispace_size_;
   int max_semi_space_size_;
   int initial_semispace_size_;
+  int target_semispace_size_;
   intptr_t max_old_generation_size_;
   intptr_t max_executable_size_;
   intptr_t maximum_committed_;