Update comment describing the length field in class Bigint.

vm/raw_object.h

Index: vm/raw_object.h
===================================================================
--- vm/raw_object.h	(revision 5801)
+++ vm/raw_object.h	(working copy)
@@ -896,14 +896,14 @@
 class RawBigint : public RawInteger {
   RAW_HEAP_OBJECT_IMPLEMENTATION(Bigint);
 
-  // Actual length at the time of allocation (later we may clamp the
-  // operational length but we need to maintain a consistent object
-  // length so that the object can be traversed during GC).
+  // Actual length in chunks at the time of allocation (later we may
+  // clamp the operational length but we need to maintain a consistent
+  // object length so that the object can be traversed during GC).
   intptr_t allocated_length_;
 
-  // Operation length of the bigint object, clamping can cause this length
-  // to be reduced. If the signed_length_ is negative then the number
-  // is negative.
+  // Operational length in chunks of the bigint object, clamping can
+  // cause this length to be reduced. If the signed_length_ is
+  // negative then the number is negative.
   intptr_t signed_length_;
 
   // A sequence of Chunks (typedef in Bignum) representing bignum digits.