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This code is disabled by default behind kDesktopPWAsWithoutExtensions and
kDesktopPWAsUSS base features.

- Implement WebAppSyncBridge::MergeSyncData and ApplySyncChanges.
- Implement "Local changes" propagation to change_processor.

Unit tests and browser tests will be added later in a follow up CL.
An MVP implementation of the bridge needed first.

This CL follows Chrome Sync's Model API doc
https://chromium.googlesource.com/chromium/src/+/HEAD/docs/sync/model_api.md

In next CLs:
Implement sync-initiated installs and uninstalls.

TBR=alancutter@chromium.org

Bug: 860583
Change-Id: I65d3cae6f47dd8fc8fdbe2c7cfb2ed13012f90f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1830494Reviewed-by: Alexey Baskakov <loyso@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Alexey Baskakov <loyso@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704482}