[Extensions] Remove old comment From XWalkExtensionProcessHost.
[Extensions] Use OnRenderProcessHostCreated to create a ExtensionProcessHost

This patch postpones the creation of a XWalkExtensionProcessHost until
a new RenderProcessHost is created. For achieving that we also had to
delay calling XWalkExtensionProcessHost::RegisterExternalExtensions.
[Extensions] Create Extensions' Infra per RenderProcess

For fully achieving it, this patch adds:
- One ExtensionProcess per RenderProcess;
- An InProcess ExtensionServer per RenderProcess;
- An InProcess MessageFilter per RenderProcess.

The patch also refactors XWalkExtensionService::OnRenderProcessHostCreated,
creating two helper functions to:
CreateExtensionProcessHost() and CreateInProcessExtensionServer().
[Extensions] Move ExtensionProcessHost, in_process server and message filter to a single struct

This patch adds a struct ExtensionData to XWalkExtensionService that will
be responsible for holding the data related to extensions per RenderProcess.

This has reduced our need of 3 maps to only one map.
[Extensions] Add test for 1-EP-per-RP
Merge pull request #939 from jeez/multi-eps

[Extensions] ExtensionProcess per RenderProcess implementation