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Writes to lossy prefs should not trigger writes to disk to be scheduled.
This behavior is implemented for JsonPrefStore. CommitPendingWrites will
ensure that all outstanding lossy writes are also written to disk.

BUG=476800

Committed: https://crrev.com/e9d8c9882ca272c803711e26ded4dd7d5203a59e
Cr-Commit-Position: refs/heads/master@{#329122}

Review URL: https://codereview.chromium.org/1127963002

Cr-Commit-Position: refs/heads/master@{#329287}