[vm] Update --log_growth to use MB to match --verbose_gc.
[vm] Update --log_growth to use MB to match --verbose_gc.

Also include the soft threshold.

TEST=no
Change-Id: I834bc62518cedff98a4d4dbd16b1b50532cd002e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/246050
Commit-Queue: Ryan Macnak <d738b7f001baf9789095d7dc549cff6de60b011d@google.com>
Reviewed-by: Siva Annamalai <2e618a8cbd7bb6b3d4409061ae407d692a098546@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk