* DisableInlineAllocationSteps was a blunt hammer added to work around
  tests that needed to avoid artificially lower limits imposed by
  observers. PauseAllocationObserversScope can properly disable step on
  a temporary basis.
* Modify tests. Remove DisableInlineAllocationSteps.

This exposed a bug in allocation observers: we were not doing a step
when a fresh page is added.

Fix this by moving the step into UpdateAllocationInfo. We should be
doing a step (and keeping top_on_previous_step_ consistent) whenever
we move move top(). UpdateAllocationInfo is the correct place for this
rather than the callers of UpdateAllocationInfo.

Bug: 
Change-Id: I2edc238dc2e73bf9a2e9738c2a9b50efcac5cbf0
Reviewed-on: https://chromium-review.googlesource.com/821052
Commit-Queue: Ali Ijaz Sheikh <ofrobots@google.com>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50052}