GitLab

Modify GetShmemTempDir to check for a runtime switch to always use the temp dir
Changes tests in shared_memory_unittest.cc to be parameterized and
test also for when /dev/shm is disabled.

Bug: 736452,792117
Change-Id: Id930ea525980e97c4ab70e37689301370776fc79
Reviewed-on: https://chromium-review.googlesource.com/810325Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: David Vallet <dvallet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523959}