Previously Context had no Impl class, but had a special relationship
with the instanced Renderer class. Having a ContextImpl backing every
Context will allow new designs to enable things like multithreading
(where each ContextImpl stores a Context-specific device) or non-
virtual Contexts on Android or other platforms where it is more
efficient.

A large refactoring patch that touches every back-end.

BUG=angleproject:1363

Change-Id: Icb73a7d37447f08a664eeb499a310ba05d71a57e
Reviewed-on: https://chromium-review.googlesource.com/342052Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>