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Add a new histogram Net.QuicSession.NetworkProbingResult which records network probing results according to different migration causes.

This is a temporary change in order to understand the success/failure
rate of network probings when attempting connection migration.

Change-Id: I3c4c64cec67322a23e39c22269bc3f44790a650b
Reviewed-on: https://chromium-review.googlesource.com/973912
Commit-Queue: Fan Yang <fayang@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Reviewed-by: Zhongyi Shi <zhongyi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546032}