[vm, gc] Emit a timeline event for NotifyLowMemory.
[vm, gc] Emit a timeline event for NotifyLowMemory.

TEST=--complete-timeline
Change-Id: I6c08753fc6ea3a98d52d288888d7707c43df4d36
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/214742
Reviewed-by: Siva Annamalai <2e618a8cbd7bb6b3d4409061ae407d692a098546@google.com>
Commit-Queue: Ryan Macnak <d738b7f001baf9789095d7dc549cff6de60b011d@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