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This CL adds metrics for how often preinstalled web apps migrate and
remove an existing app install.

This data is useful to monitor rollouts of default web apps that are
replacing old Chrome apps to ensure the migration is happening as
expected.

Bug: 809304, 1058265
Change-Id: Ie9edb4a1cf89e84d7e56a20bad028dd8ea1aeb07
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2561999
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Glen Robertson <glenrob@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#832148}