X64: Update allocation to work with no scratch registers at all.

src/x64/macro-assembler-x64.h

Index: src/x64/macro-assembler-x64.h
diff --git a/src/x64/macro-assembler-x64.h b/src/x64/macro-assembler-x64.h
index 59b19fdadaa25aa38dd4c63f62ddfa3565e968d6..0bfe73031c14d912227cfc75718cc12f187cc207 100644
--- a/src/x64/macro-assembler-x64.h
+++ b/src/x64/macro-assembler-x64.h
@@ -770,10 +770,17 @@ class MacroAssembler: public Assembler {
   void LeaveFrame(StackFrame::Type type);
 
   // Allocation support helpers.
+  // Loads the top of new-space into the result register.
+  // If flags contains RESULT_CONTAINS_TOP then result_end is valid and
+  // already contains the top of new-space, and scratch is invalid.
+  // Otherwise the address of the new-space top is loaded into scratch (if
+  // scratch is valid), and the new-space top is loaded into result.
   void LoadAllocationTopHelper(Register result,
                                Register result_end,
                                Register scratch,
                                AllocationFlags flags);
+  // Update allocation top with value in result_end register.
+  // If scratch is valid, it contains the address of the allocation top.
   void UpdateAllocationTopHelper(Register result_end, Register scratch);
 };