Add some interfaces to the GC that allow us to reserve space. This is needed...

src/mark-compact.h

Index: src/mark-compact.h
===================================================================
--- src/mark-compact.h	(revision 3575)
+++ src/mark-compact.h	(working copy)
@@ -92,7 +92,9 @@
   static bool HasCompacted() { return compacting_collection_; }
 
   // True after the Prepare phase if the compaction is taking place.
-  static bool IsCompacting() { return compacting_collection_; }
+  static bool IsCompacting() {
+    return state_ != IDLE && compacting_collection_;
+  }
 
   // The count of the number of objects left marked at the end of the last
   // completed full GC (expected to be zero).