The data size & block member info was getting messy, so clean up
how we query this from the Impl layer.

Also remove the register information from gl::UniformBlock, moving
it into the D3D-only world.

BUG=angleproject:1172

Change-Id: I40af658ebbd6b7c1a4251906a387ebcbb621cf77
Reviewed-on: https://chromium-review.googlesource.com/304150Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tryjob-Request: Jamie Madill <jmadill@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>