Unglobalize browserContext

This patch makes it possible to have more than one BrowserContext,
for instance for off-the-record browsering but also to enable any
other configuration on browser context level.

This will make it possible to add page-groups settings and API for
settingsthat can only be done on BrowserContext level.

This patch does not expose any new API.

Change-Id: I0a0e194449cfb5a096a4ac6e2f2b3882b6b4cfa2
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>