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We have some crash reports for Linux that tell us that processes are
sometimes getting killed with SIGBUS when they write to memory mapped
to shared memory file. The suspected cause is /dev/shm volume getting
full.

This CL is an attempt to avoid this by using posix_fallocate() instead
of ftruncate(), which preallocates memory early and detects disk full
before we write to memory.

The code is only enabled for Linux, because we currently see the symptom
happening only on Linux.

Bug: 1014296
Change-Id: I45c5f4152639a742da689472b73c5e1dd717a2d7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1895028Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712524}