Replace HeapNumber as doublebox with an explicit MutableHeapNumber.

include/v8.h

Index: include/v8.h
diff --git a/include/v8.h b/include/v8.h
index 608e3c52c348ec5a39da65f75503c27abf79b44e..de2c9aa528d6b3898b9f9215c036aa37a5ce9c48 100644
--- a/include/v8.h
+++ b/include/v8.h
@@ -5571,7 +5571,7 @@ class Internals {
   static const int kNullValueRootIndex = 7;
   static const int kTrueValueRootIndex = 8;
   static const int kFalseValueRootIndex = 9;
-  static const int kEmptyStringRootIndex = 154;
+  static const int kEmptyStringRootIndex = 155;
 
   static const int kNodeClassIdOffset = 1 * kApiPointerSize;
   static const int kNodeFlagsOffset = 1 * kApiPointerSize + 3;
@@ -5582,10 +5582,10 @@ class Internals {
   static const int kNodeIsIndependentShift = 4;
   static const int kNodeIsPartiallyDependentShift = 5;
 
-  static const int kJSObjectType = 0xbb;
+  static const int kJSObjectType = 0xbc;
   static const int kFirstNonstringType = 0x80;
   static const int kOddballType = 0x83;
-  static const int kForeignType = 0x87;
+  static const int kForeignType = 0x88;
 
   static const int kUndefinedOddballKind = 5;
   static const int kNullOddballKind = 3;