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Unsent metrics logs get trimmed if they're larger than a preset size.
This change implements a new UMA metrics histogram called
"UMA.UnsentLogs.DroppedSize" to record the size of each of these trimmed
logs.

Also adds a unit test case for a similar existing histogram that
measures dropped log counts.

Bug: None
Test: metrics_unittests
Change-Id: If6ed17a2a1327c2bd3d3a9da16926b91e72a6f43
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2482322
Commit-Queue: Akira Baruah <akirabaruah@google.com>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819434}