MIPS: Fix in-object memory slack tracking bug.

src/mips/builtins-mips.cc

Index: src/mips/builtins-mips.cc
diff --git a/src/mips/builtins-mips.cc b/src/mips/builtins-mips.cc
index e95c42292ef4bd8e21bcf7eb40ae9afbceceff69..326b9e44999406e4c58a869444c36d2d6e892b24 100644
--- a/src/mips/builtins-mips.cc
+++ b/src/mips/builtins-mips.cc
@@ -433,9 +433,7 @@ static void Generate_JSConstructStubHelper(MacroAssembler* masm,
         __ CallRuntime(Runtime::kFinalizeInstanceSize, 1);
 
         __ Pop(a1, a2);
-        // Slack tracking counter is Map::kSlackTrackingCounterEnd after runtime
-        // call.
-        __ li(t2, Map::kSlackTrackingCounterEnd);
+        __ li(t2, Operand(Map::kSlackTrackingCounterEnd - 1));
 
         __ bind(&allocate);
       }