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With this change, LiteScriptShownToUser and LiteScriptFinished UKM
metrics will now be recorded with the native coordinator. Unfortunately,
the other two metrics (LiteScriptOnboarding and LiteScriptStarted) are
still in Java for now.

Recording the UKMs in native allows for better unit tests and tighter
control of when to emit those events.

This CL also fixes a bug which surfaced during testing: navigation
events while the web_contents are invisible should be ignored.

This CL does not add any new UKMs. The existing set of UKMs was
approved in crbug.com/1110887 in the privacy section of the design doc.

Note that this is not yet hooked up to prod. The relevant CL is
currently under review, here: http://crrev/c/2524524

Bug: b/172548211
Change-Id: I93cdfc2ac52a33d63247f967063d34a353924833
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2521810
Commit-Queue: Clemens Arbesser <arbesser@google.com>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Marian Fechete <marianfe@google.com>
Cr-Commit-Position: refs/heads/master@{#825397}