This class can contain impl-specific functionality for a Context.
This will eventually replace the Renderer class, and we can then
start passing around a gl::Context instead of gl::ContextState.

In D3D11, the ContextImpl could hold a DeferredContext, which would
enable multi-thread rendering. In GL, we can implement non-virtual
(native) Contexts. In Vulkan it might store the logical device.

BUG=angleproject:1363

Change-Id: I39617e6d1a605d1a9574832e4d322400b09867ec
Reviewed-on: https://chromium-review.googlesource.com/340745Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>