We were starting an allocation step during NewSpace::AddFreshPage. At
this point, we had advanced the page, but not updated allocation_info_.
This ultimately led to assertions as Space::Size was not expecting
to be called when to_space_.page_{high,low} are inconsistent with
allocation_info_.top().

The solution here is to avoid starting the step in the middle of the
space state mutation. We account for memory allocated so far before the
mutation is started, and then start a new step after the mutation has
been completed.

Bug: chromium:806179
Change-Id: I17ee896d80c4ec752baa2b17c3fd2bef7ea2ca33
Reviewed-on: https://chromium-review.googlesource.com/889981Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ali Ijaz Sheikh <ofrobots@google.com>
Cr-Commit-Position: refs/heads/master@{#50932}