This metadata class captures some of our commonly referenced but also
complex flats into a shared place. We can then re-use them in the
semantic code, the DynamicHLSL linking code, and the program code.

Refactoring patch only.

BUG=angleproject:1202

Change-Id: I8b6088cfa5488c5173a6f06c15abab5a4ead4cb8
Reviewed-on: https://chromium-review.googlesource.com/311700
Tryjob-Request: Jamie Madill <jmadill@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>