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This CL refactors WebAppInstallFinalizer database write code to not
plumb through bool is_new_install. Instead the database write callsites
encode what they want to happen afterwards as part of the CommitCallback.

This is a refactor only CL in preparation for:
https://chromium-review.googlesource.com/c/chromium/src/+/2216143

Bug: 1084405
Change-Id: Ib71740878d9e339fd8cc6ee73b6bd6fa1b782272
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2217835
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Alexey Baskakov <loyso@chromium.org>
Cr-Commit-Position: refs/heads/master@{#773068}