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 7d27029d23c8ee403588f88f8b4a3abe87a4fa4d..f734b9a44f0873a8265413053e848a58cda51316 100644
--- a/include/v8.h
+++ b/include/v8.h
@@ -5586,7 +5586,7 @@ class Internals {
   static const int kNullValueRootIndex = 7;
   static const int kTrueValueRootIndex = 8;
   static const int kFalseValueRootIndex = 9;
-  static const int kEmptyStringRootIndex = 160;
+  static const int kEmptyStringRootIndex = 161;
 
   // The external allocation limit should be below 256 MB on all architectures
   // to avoid that resource-constrained embedders run low on memory.
@@ -5601,10 +5601,10 @@ 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;