New helper functions are added for collecting built-in variables, and
the traverser is encapsulated inside VariableInfo.cpp. The helper
functions get data for built-in variables from the symbol table, so a
duplicate copy of the data doesn't need to be maintained in
CollectVariables any more.

BUG=angleproject:2068
TEST=angle_unittests

Change-Id: I42595d0da0e5d4fb634a3d92f38db1dd6dd9efab
Reviewed-on: https://chromium-review.googlesource.com/549323Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>