Revert "Replace HeapNumber as doublebox with an explicit MutableHeapNumber."

src/mips/stub-cache-mips.cc

Index: src/mips/stub-cache-mips.cc
diff --git a/src/mips/stub-cache-mips.cc b/src/mips/stub-cache-mips.cc
index e3d8fe14d939bac5a876a8cb580631acd97536ce..16c69d57addad40d22644f29281a48ba7c8e6bb1 100644
--- a/src/mips/stub-cache-mips.cc
+++ b/src/mips/stub-cache-mips.cc
@@ -413,9 +413,8 @@ void StoreStubCompiler::GenerateStoreTransition(MacroAssembler* masm,
     }
   } else if (representation.IsDouble()) {
     Label do_store, heap_number;
-    __ LoadRoot(scratch3, Heap::kMutableHeapNumberMapRootIndex);
-    __ AllocateHeapNumber(storage_reg, scratch1, scratch2, scratch3, slow,
-                          TAG_RESULT, MUTABLE);
+    __ LoadRoot(scratch3, Heap::kHeapNumberMapRootIndex);
+    __ AllocateHeapNumber(storage_reg, scratch1, scratch2, scratch3, slow);
 
     __ JumpIfNotSmi(value_reg, &heap_number);
     __ SmiUntag(scratch1, value_reg);