Add migration code to change existing domain scoped content settings to be origin scoped

Migrate existing content settings of cookies, images, javascript, plugins,
popups, mouselock and automatic-downloads to be origin scoped so that their
settings only affect origins.

This is the first step of migrating domain scoped settings to be origin scoped.
In the following CLs we will:

- Add callbacks in sync layer so that the migrated old settings are cleared
after the sync process.

- Change the scoping type of these content types so that new generated settings
are all origin scoped and hook up all the migration code and sync callbacks.


Cr-Commit-Position: refs/heads/master@{#404340}
6 files changed