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- connection will not be closed if there's any packet write error when
  sending connectivity probes as those are sent on a different path;
- connection will not be blocked if the connectivity probe is sent by a
  non-default packet writer.
Flag protected by quic_handle_write_results_for_connectivity_probe.

Manually merge internal change: 189676976.

Change-Id: Ie0389f3342365ed93d1c12c414e90a5118a004de
Reviewed-on: https://chromium-review.googlesource.com/972349Reviewed-by: Ryan Hamilton <rch@chromium.org>
Commit-Queue: Zhongyi Shi <zhongyi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544809}