MIPS: Fix in-object memory slack tracking bug.

src/mips64/builtins-mips64.cc

Index: src/mips64/builtins-mips64.cc
diff --git a/src/mips64/builtins-mips64.cc b/src/mips64/builtins-mips64.cc
index 30a93dc5eb7184a69aa0889afb89bd57e59a2509..e97cca21d1756f3fc11df12b95b21a5fcf857a1a 100644
--- a/src/mips64/builtins-mips64.cc
+++ b/src/mips64/builtins-mips64.cc
@@ -434,9 +434,7 @@ static void Generate_JSConstructStubHelper(MacroAssembler* masm,
         __ CallRuntime(Runtime::kFinalizeInstanceSize, 1);
 
         __ Pop(a1, a2);
-        // Slack tracking counter is Map::kSlackTrackingCounterEnd after runtime
-        // call.
-        __ li(a6, Map::kSlackTrackingCounterEnd);
+        __ li(a6, Operand(Map::kSlackTrackingCounterEnd - 1));
 
         __ bind(&allocate);
       }