Replace HeapNumber as doublebox with an explicit MutableHeapNumber.

src/ia32/stub-cache-ia32.cc

Index: src/ia32/stub-cache-ia32.cc
diff --git a/src/ia32/stub-cache-ia32.cc b/src/ia32/stub-cache-ia32.cc
index 1a745c7b7f32a7659e5a96d208c8060b0fb5393d..8321fa8d7d5ffc18ecc8caebe8c45c782cfb7f80 100644
--- a/src/ia32/stub-cache-ia32.cc
+++ b/src/ia32/stub-cache-ia32.cc
@@ -531,7 +531,7 @@ void StoreStubCompiler::GenerateStoreTransition(MacroAssembler* masm,
     __ JumpIfSmi(value_reg, miss_label);
   } else if (representation.IsDouble()) {
     Label do_store, heap_number;
-    __ AllocateHeapNumber(storage_reg, scratch1, scratch2, slow);
+    __ AllocateMutableHeapNumber(storage_reg, scratch1, scratch2, slow);
 
     __ JumpIfNotSmi(value_reg, &heap_number);
     __ SmiUntag(value_reg);