Fix sandboxed renderer coverage dumping (again).
Fix sandboxed renderer coverage dumping (again).

Follow-up to https://codereview.chromium.org/1219893002

Repeatly trying smaller buffer sizes for setsockopt doesn't work for our
purposes. The kernel will quietly set the maximum allowed buffer size if
the specified size is too large. Instead, set the desired size and then
read it back via getsockopt.

Also, SO_RCVBUF is no longer set since that does nothing for AF_UNIX
sockets.

BUG=501304
R=mdempsky@chromium.org,jln@chromium.org,inferno@chromium.org,glider@chromium.org

Review URL: https://codereview.chromium.org/1213003003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#337428}

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