[vm, gc] Honor HeapSizeFromClass not touching the header for FreeListElement and ForwardingCorpse.

TEST=ci
Change-Id: I5623f4f83c9898053fa057f666096a81b06d8d3d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/214864
Reviewed-by: Siva Annamalai <[email protected]>
Commit-Queue: Ryan Macnak <[email protected]>

Author: rmacnak-google

runtime/vm/heap/become.h

@@ -27,8 +27,9 @@ class ForwardingCorpse {
   ObjectPtr target() const { return target_; }
   void set_target(ObjectPtr target) { target_ = target; }
 
-  intptr_t HeapSize() {
-    intptr_t size = UntaggedObject::SizeTag::decode(tags_);
+  intptr_t HeapSize() { return HeapSize(tags_); }
+  intptr_t HeapSize(uword tags) {
+    intptr_t size = UntaggedObject::SizeTag::decode(tags);
     if (size != 0) return size;
     return *SizeAddress();
   }

runtime/vm/heap/freelist.h

@@ -28,8 +28,9 @@ class FreeListElement {
 
   void set_next(FreeListElement* next) { next_ = next; }
 
-  intptr_t HeapSize() {
-    intptr_t size = UntaggedObject::SizeTag::decode(tags_);
+  intptr_t HeapSize() { return HeapSize(tags_); }
+  intptr_t HeapSize(uword tags) {
+    intptr_t size = UntaggedObject::SizeTag::decode(tags);
     if (size != 0) return size;
     return *SizeAddress();
   }

runtime/vm/raw_object.cc

@@ -222,13 +222,13 @@ intptr_t UntaggedObject::HeapSizeFromClass(uword tags) const {
     case kFreeListElement: {
       uword addr = UntaggedObject::ToAddr(this);
       FreeListElement* element = reinterpret_cast<FreeListElement*>(addr);
-      instance_size = element->HeapSize();
+      instance_size = element->HeapSize(tags);
       break;
     }
     case kForwardingCorpse: {
       uword addr = UntaggedObject::ToAddr(this);
       ForwardingCorpse* element = reinterpret_cast<ForwardingCorpse*>(addr);
-      instance_size = element->HeapSize();
+      instance_size = element->HeapSize(tags);
       break;
     }
     case kWeakSerializationReferenceCid: {