top_on_previous_step_ can only be valid when Allocation Observers are
active. Add some assertions in the code to ensure this holds.

Use AllocationObserversActive() more pervasively. Remove some code based
on the established invariant.

Bug: 
Change-Id: I7f0d4c4f617ed9fa05c6b94202a90953fbc33cfd
Reviewed-on: https://chromium-review.googlesource.com/823576Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ali Ijaz Sheikh <ofrobots@google.com>
Cr-Commit-Position: refs/heads/master@{#50088}