Do not start a new step when an existing step is in progress. We may
have partially updated information as part of the current step, and the
next step will assume consistency. A new step will be started once the
current in-progress step completes.

BUG=v8:7313

Change-Id: I4c0c47c4f4b5f8b9139be24408440189679b38dc
Reviewed-on: https://chromium-review.googlesource.com/882507Reviewed-by: Ali Ijaz Sheikh <ofrobots@google.com>
Commit-Queue: Ali Ijaz Sheikh <ofrobots@google.com>
Cr-Commit-Position: refs/heads/master@{#50855}