Refactor CollectVariables 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/549323 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>