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Mostly this reworks the sync change processing order to try to declare
variables near their first use, factor out common bits, and reduce
indenting.  It does change the error message on ACTION_INVALID
(shouldn't be a problem).

Bug: none
Change-Id: I843a76353bae9cba6f693f996ab60deec1b01e4a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2145913
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758354}