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This CL fixes a bug in commit b2454381 "Add new ExtensionManagement preference". The default value of IndividualSettings are supposed to be |default_settings_| instead of fallback value for each settings.
This bug was not exposed in unit tests since IndividualSettings contains only installation mode settings now.

BUG=177351

Review URL: https://codereview.chromium.org/593223003

Cr-Commit-Position: refs/heads/master@{#296475}