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A previous change added error handling for when prefs serialization
fails.  The check call will result in a crash so I am updating the
comment above it to reflect that and remove the unreachable code
below it.

Change-Id: If16aff75e100c3a92e27344203ccad6d292caf5d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1856878
Commit-Queue: Chris Davis <chrdavis@microsoft.com>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705242}