Implements layoutTestController.overridePreference()

This change provides the infrastructure for
layoutTestController.overridePreference(). Currently, we only provide an
implementation for the preference 'WebKitOfflineWebApplicationCacheEnabled',
which is required by the layout test http/tests/appcache/disabled.html.

Change-Id: I8552f2f4e23b982db2d067ffa20c052e56d8fb7f