[vm] Per isolate group roots accessed via TLS.

Remove special case for null in message snapshots; snapshots are sometimes read or written with no current isolate group.

Currently still all copies pointing into the VM isolate.

TEST=ci
Change-Id: I6fa47b2c04b0abaae74210c86f9fdf17ec932db7
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/493222
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

runtime/vm/dart_api_message.h

@@ -17,9 +17,6 @@ namespace dart {
 
 // This class handles translation of certain ObjectPtrs to CObjects for
 // NativeMessageHandlers.
-//
-// TODO(zra): Expand to support not only null, but also other VM heap objects
-// as well.
 class ApiObjectConverter : public AllStatic {
  public:
   static bool CanConvert(const ObjectPtr raw_obj) {