Add migration code for the EnabledPlugins and DisabledPlugins policies.

These policies are deprecated in favor of:
DefaultPluginSettings (aka. Flash BLOCK/ALLOW)
AlwaysOpenPdfExternally (aka. PDF plugin disabled)

BUG=654731
TEST=browser_tests:PolicyPrefIndicator

Review-Url: https://codereview.chromium.org/2410113002
Cr-Commit-Position: refs/heads/master@{#425743}
5 files changed