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For remote connections, SHM doesn't work, but the MIT-SHM extension may
still be available, so we need to do an explicit check before trying to
use SHM.

BUG=1035803

Change-Id: I189ab94b82922572e52b717dc2043b8404745dec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1980971
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#728564}