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The new WebAppMigrationManager is responsible for any migration-related
code.

WebAppMigrationManager::StartDatabaseMigration performs the offline
migration of all bookmark apps to new web apps registry.

We do the offline migration only once, before the initial sync.

Any additional apps arriving after the initial sync should be installed
via WebAppSyncBridge::CommitUpdate() "as if" a user installs them.
This will be explained in go/chrome-bmo-migration doc and implemented
as a follow up CL.

WebAppMigrationManagerBrowserTest demonstrates some new techniques
on how to write integration tests:
- PRE test and content::IsPreTest().
- URLLoaderInterceptor instead of EmbeddedTestServer.

More tests and test/data/ will follow:
- DatabaseMigration_FullManifest and manifest_full_data.json.
- A set of apps to test the queue.

Bug: 1020037
Change-Id: I0038d321932a72deeb00fd21cd3171f1b8ee0fcb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2014340
Commit-Queue: Alexey Baskakov <loyso@chromium.org>
Reviewed-by: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742076}