[Extensions] Refactoring XWalkExtensionService::RegisterExtension

Nowadays our XWalkExtensionService is only capable of handling
one RenderProcess. Thus, it only has one in_process XWalkExtensionServer
for handling internal extensions. Soon enough this will considerably
change and we will have to be capable of handling one in_process
XWalkExtensionServer per RenderProcess.

This patch is the first step towards this: internal extensions
will be created directly by the XWalkExtensionService::Delegate
"on-demand" every time a new in_process XWalkExtensionServer
is created for a new RenderProcess and it is time to register
the internal extensions on it.

The entity implementing XWalkExtensionService::Delegate is
XWalkBrowserMainParts. This way we keep the old behavior and design
not polluting XWalkExtensionService.

Necessary adjustments were made to RegisterExtensionsCallback and
browser_tests for the same reason.