This also reformats the Sampler class to use a shared state struct
with the implementation. It removes the call to sync the sampler
state from the StateManagerGL::setGenericShaderState method, since
it should all be handled at the front-end now.

Also rename 'syncImplState' to 'syncState' methods.

BUG=angleproject:1387

Change-Id: I5f0219b719aee99aaaa486ec188b2af0c9128e6a
Reviewed-on: https://chromium-review.googlesource.com/648054
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>