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This CL adds logging of proposed Translate proto to UKM. This proto
captures a summary of the user's interaction with Translate from the
beginning of the page load to when the proto is logged. In the event
that the user backgrounds Chrome and later returns, there may be more
than one proto logged per page load. In this case the sequence number of
each proto can be used distinguish the order that the protos were
logged.

Full design doc: https://docs.google.com/document/d/1dyWh1Xw5VgUA00VA-5PTgKQ6ItziPBnSyeDR8saJ9vM/edit?usp=sharing
UKM collection review: https://docs.google.com/document/d/1UNVJ2HC35Cy2C6zCb6DdwKFhafURGeV9gDzY5Jm5id8/edit?usp=sharing

Bug: 1114868
Change-Id: If970d2b656c7355e6a05fa06729978265d9ecb64
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2590754
Commit-Queue: Max Curran <curranmax@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Reviewed-by: Scott Little <sclittle@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841292}