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This records the network connection type any time it changes.

Also adds an enum histogram value for the new 5G type that was
recently added to the ConnectionType enum.

Bug: 1127134
Change-Id: I5a7d1306b69c4714a8b24a5462871aa0842121d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2431589
Commit-Queue: Ken Rockot <rockot@google.com>
Reviewed-by: Brian White <bcwhite@chromium.org>
Reviewed-by: Zhongyi Shi <zhongyi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811453}