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By default open() blocks when opening a fifo until the writer also
opens the fifo. In the past AudioPipeReader was using O_NONBLOCK
when opening the pipe for reading, but that was changed in r277315,
and may result in host hanging if pulseaudio doesn't open the pipe
for some reason. Fixed AudioPipeReader to always pass O_NONBLOCK
to open().

BUG=420090
R=lambroslambrou@chromium.org

Review URL: https://codereview.chromium.org/820123002

Cr-Commit-Position: refs/heads/master@{#309592}