[Extensions] Add an external extensions XWalkExtensionClient to RendererController

XWalkExtensionClient::Initialize(IPC::Sender*) was added so we can:
  1- create a XWalkExtensionClient;
  2- initialize a SyncChannel using the Client as its listener;
  3- initialize the Client by passing the SyncChannel as its Sender.

This change follows exactly how XWalkExtensionServer is designed nowadays.
[Extensions] Don't register external extensions in BP if using EP
Merge pull request #673 from jeez/eph-channel-client

[Extensions] Create a XWalkExtensionClient for external extensions