Don't start new steps recursively if a step is already in progress.
Having this property on a space is not sufficient, as an allocation
is a global (heap-wide) event. Computing the next step size, for example
is a property of all observers in existence rather than the spaces in
existence.

In this case a failure was due to the fact that we attempted to compute
the next step size while a given observer was mid-way through its step
triggered from a different space. bytes_to_next_step_ was partially
updated at that point.

BUG=v8:7313

Change-Id: Iaf632fce2cfd5ed49b0f41a69c3694e505e17d81
Reviewed-on: https://chromium-review.googlesource.com/887174
Commit-Queue: Ali Ijaz Sheikh <ofrobots@google.com>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50909}