Reserve code range block for evacuation.

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}