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Chrome OS SplitSettingsSync has a separate control for syncing OS data
types that isn't controlled by the browser's sync-the-feature. Change
ModelType::WEB_APPS to run using the transport layer when
kSplitSettingsSync is enabled and the OS sync feature is on.

Manual test:
Run chrome --enable-features=SplitSettingsSync,SyncManualStartChromeOS,
  DesktopPWAsWithoutExtensions,DesktopPWAsUSS
Add an account
Enable OS sync in the first-run dialog
Verify chrome://sync-internals shows Web Apps are syncing, even though
browser sync is off.

Also added automated tests to sync_integration_tests.

Bug: 1013466, 1031549
Change-Id: I2cdb4ccf6b4027ce45044d945f02913482ad239d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1970290
Commit-Queue: Marc Treib <treib@chromium.org>
Auto-Submit: James Cook <jamescook@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#725457}