[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.