Re-implement UserAgent in browser

- Remove calls to SWE's WebSetting API and use the newer implementation of
  the engine. The newer implementation tracks UserAgent per navigation
  entry.
- Allow to override the UserAgent using command line switches.
- Remove debug option to change UserAgent string.
- BrowserConfig class cleanup.

Change-Id: Ib1cc4763e0cbf0207793d1f4e6b1644a9dbb5c08
10 files changed