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This is a reland of
https://chromium-review.googlesource.com/c/chromium/src/+/1914005 CL.

The problem was that two IsTrackingMetadata() calls were under DCHECK macro.
Those calls didn't happen in release builds with `dcheck_always_on=false`
GN arg.

The fix: promote DCHECK to CHECK to make it debug/release/dcheck_always
agnostic.

Original description:

Improve WebAppSyncBridge::UpdateSync to use both current_state (before commit)
and new_state (to be commit).

In the future, it will help to skip local updates which don't touch sync data.

Add WebAppSyncBridge::CommitUpdate unit tests (a.k.a. Local Changes).

These tests is a promised follow up for this CL:
https://chromium-review.googlesource.com/c/chromium/src/+/1830494

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

TBR=treib@chromium.org

Bug: 860583
Change-Id: I80a657c0d595bc03bebb8e241b1866daf5aa077e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1915647Reviewed-by: Alexey Baskakov <loyso@chromium.org>
Commit-Queue: Alexey Baskakov <loyso@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715207}