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This CL adds three metrics for preinstalled web apps:
 - WebApp.Preinstalled.ConfigErrorCount
 - WebApp.Preinstalled.EnabledCount
 - WebApp.Preinstalled.DisabledCount

This helps track whether we see the expected number of preinstalled apps
configured on devices and whether there are bad config files.

Bug: 1128801
Change-Id: I2a33d9d5812ff29d0218f345ba68c8727542218c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2418756
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Glen Robertson <glenrob@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809303}