[Extensions] Add dummy ExtensionProcessHost to ExtensionService
[Extensions] Add ExtensionProcess spawned by ExtensionProcessHost.
[Extensions] Call RegisterExtensions from ExtensionProcess
[Extensions] Add a XWalkExtensionServer to XWalkExtensionProcess

A dummy IPC::Sender was added for testing purposes while we don't
handle the creation of our IPC Channel.
[Extensions] Use RegisterExternalExtensionsInDirectory in XWalkExtensionProcess
[Extensions] Fixing Windows build for launching ExtensionProcess
[Extensions] Fixes after review for ExtensionProcess and ExtensionProcessHost

Mainly here I have:
- added comments;
- removed unneeded loops from ExtensionProcess;
- guarded the creation of ExtensionProcessHost by the --enable-extension-process switch;
- changed to VLOG(1).
[Extensions] Turn off sandboxing for ExtensionProcess in Windows

We don't want that for a process that is supposed to have access
to the entire platform...
Merge pull request #634 from jeez/eph

Add ExtensionProcess and ExtensionProcessHost