[Extensions] Create the EP<->RP IPC Channel

We create a MODE_SERVER Channel on the ExtensionProcess
and use the BrowserProcess (through ExtensionProcessHost)
to let the RenderProcess know about this ChannelHandle.
[Extension] Create a Client Channel for EP on RenderProcess

For now this MODE_CLIENT channel will use a dummy listener to
receive its messages. It will later be substituted by a
XWalkExtensionClient.
[Extensions] Use the ExtensionsServer from ExtensionProcess to communicate with RP

Now we are using the EP <-> RP IPC Channel to make the ExtensionServer,
from the ExtensionProcess, communicate with its DummyListener running on the
renderer process. We also make it only send the RegisterExtension message
after the RenderProcess has connected to its channel.
[Extensions] Fixing ExtensionProcess channel creation after review

- Adding a function to XWalkExtensionProcessHost that sends the
EP channel handle when ready and it's used by OnRenderProcessHostCreated
and OnRenderChannelCreated;

- Splitting XWalkExtensionProcess::CreateChannel() into 2 more specific
functions: CreateBrowserProcessChannel() and CreateRenderProcessChannel();

- Fixing style issues.
Merge pull request #663 from jeez/eph-channel

[Extensions] Create the EP <-> RP IPC Channel