Add an interface to Space, so Heap no longer needs to be friend of Space.

Also, fix and add comments with respect to utilization.

Change-Id: Ia9d27cc16d2b85ac97a48fee80567829f41224f4
diff --git a/src/heap.h b/src/heap.h
index d847a87..c25803f 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -92,8 +92,9 @@
   static void SetTargetHeapUtilization(float target) {
     target_utilization_ = target;
   }
+  // Sets the maximum number of bytes that the heap is allowed to allocate
+  // from the system.  Clamps to the appropriate maximum value.
   static void SetIdealFootprint(size_t max_allowed_footprint);
-  static void SetSoftLimit(size_t soft_limit);
 
   // Blocks the caller until the garbage collector becomes idle.
   static void WaitForConcurrentGcToComplete();