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When the zygote starts it sends a "boot message" to the browser process
across a socket. If the browser has already exited or crashed the
sendmsg() will fail. We've seen this happen in the Chrome OS autotest
lab.

Instead of crashing the zygote, just log an error and cleanly exit.

BUG=692227
TEST=add sleep to zygote startup, kill browser before zygote boot
message is sent, verify zygote shuts down cleanly

Review-Url: https://codereview.chromium.org/2851473005
Cr-Commit-Position: refs/heads/master@{#468027}