[heap] Introduce HeapVisitor interface.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 00144d2ae31c118e445f0bd5c4fe2e2300c1a666..33841403a3b3abc31dc668a65ab367f20bcf8981 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -2893,6 +2893,8 @@ class FixedDoubleArray: public FixedArrayBase {
   DECLARE_PRINTER(FixedDoubleArray)
   DECLARE_VERIFIER(FixedDoubleArray)
 
+  class BodyDescriptor;
+
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(FixedDoubleArray);
 };
@@ -3231,6 +3233,8 @@ class ByteArray: public FixedArrayBase {
   // Maximal length of a single ByteArray.
   static const int kMaxLength = kMaxSize - kHeaderSize;
 
+  class BodyDescriptor;
+
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(ByteArray);
 };
@@ -8488,6 +8492,8 @@ class SeqOneByteString: public SeqString {
   static const int kMaxSize = 512 * MB - 1;
   STATIC_ASSERT((kMaxSize - kHeaderSize) >= String::kMaxLength);
 
+  class BodyDescriptor;
+
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(SeqOneByteString);
 };
@@ -8528,6 +8534,8 @@ class SeqTwoByteString: public SeqString {
   STATIC_ASSERT(static_cast<int>((kMaxSize - kHeaderSize)/sizeof(uint16_t)) >=
                String::kMaxLength);
 
+  class BodyDescriptor;
+
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(SeqTwoByteString);
 };