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On Windows the default UDP send buffer size is 8192. OSX default is
9216. On Linux default is 212992. The values are too small on Windows
and OSX. This cannot make good use of the A-MPDU of 802.11n (64KB),
which is the most common network for Cast Streaming.

We have also seen strong evidence that the default buffer size is
limiting, e.g. long packet queuing delay on Windows, p2p socket seeing
a lot of EWOULDBLOCKs.

This code uses pacer maximum burst size (in bytes) as the send buffer
size. Also added an option "send_buffer_min_size" to allow application
to choose an even larger value for send buffer size.

BUG=423545

Review URL: https://codereview.chromium.org/654843007

Cr-Commit-Position: refs/heads/master@{#300183}