[vm] Add missing safepoint transition to DartAPI_Multiroot_Valid.
[vm] Add missing safepoint transition to DartAPI_Multiroot_Valid.

Change-Id: Icbe3d9a1c810e3afe34a3e9a4990fbc6e169244b
Reviewed-on: https://dart-review.googlesource.com/72940
Reviewed-by: Zach Anderson <9f8b31e960a68dade71618f1ff6da6e23a1edfc9@google.com>
Commit-Queue: Ryan Macnak <d738b7f001baf9789095d7dc549cff6de60b011d@google.com>

dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk