Mechanism to clear data through "Reset to default" setting

- Added hooks in BrowserYesNoPreference to support a third
  button.
- Enabled the third button for "Reset to default" setting
- If user clicks on this new button, it'll also clear the
  private data (history, cache, etc)

Change-Id: I7b224805d2ab8ef440a13948821c871b0b4a791e
5 files changed