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Previously we did in-place immediate updates for existing apps.
This was not consistent with the commit ideology since effects were becoming
visible immediately. It didn't match the creation and deletion of apps: they
were deferred until commit.

With this CL we implement basic copy-on-write for WebApp object updates.
We commit the created copy into the original for real, during the commit call.

In next CLs: Fix UpdateSync algorithm to avoid excessive change_processor
Put and Delete calls.

TBR=alancutter@chromium.org

Bug: 860583
Change-Id: I26eb53b668f74ce0c606a60af9016aa8a4cca1c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1851792
Commit-Queue: Alexey Baskakov <loyso@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705007}