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This adds a class that holds a log of events related to a single
app's push install process. The class is not hooked up to anything
yet, so no log entries are read or written.

Bug: b/73277923
Test: unit_tests
Change-Id: I046f5c4871e1ae8867c96c3f6bd5c6cee31b8e01
Reviewed-on: https://chromium-review.googlesource.com/915924
Commit-Queue: Bartosz Fabianowski <bartfab@chromium.org>
Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537639}