Move methods and member variables that generalize to different types
of field lists into TFieldListCollection and put implementations of
methods in cpp files.

This prepares for making TStructure inherit from TSymbol.

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: I63095242dd17aac2d2efd616b49be1143cfc1f92
Reviewed-on: https://chromium-review.googlesource.com/793813Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>