This allows the dirty objects to notify the context if something
needs to be communicated down to the ContextImpl. In this case it
means a dirty Texture can notify the Context that it needs to re-apply
texture bindings.

BUG=angleproject:1387

Change-Id: I162115e51112d1c27c0dab621d1b3d14446af96d
Reviewed-on: https://chromium-review.googlesource.com/648052Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>