Reserve code range block for evacuation.

If we run out of code range, then GC wouldn't be able to compact code space,
because it wouldn't be able to allocate a new page. This can cause code space
fragmentation and OOM crashes.

BUG=chromium:430118
LOG=Y

Review URL: https://codereview.chromium.org/742733002

Cr-Commit-Position: refs/heads/master@{#25441}