This change adds DCHECKs for calls such as mprotect, as well as marking some of
the memory allocation and deallocation routines as V8_MUST_USE_RESULT. This
additional checking gives us more useful information for failure in the presence
of, for example, address space exhaustion.

Bug: 
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I5bc76c1da6160262d3d556fea49d284ddd4e02c5
Reviewed-on: https://chromium-review.googlesource.com/721267
Commit-Queue: Eric Holk <eholk@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49164}