Use TLValueTrackingTraverser in ValidateLimitations Use TLValueTrackingTraverser to determine whether a loop index is used as an l-value. This replaces custom logic in ValidateLimitations, greatly simplifying the code. Also pass the symbol table to ValidateLimitations as a parameter, which removes the need to store a global pointer to the current ParseContext. BUG=angleproject:1960 TEST=angle_unittests, WebGL conformance tests Change-Id: I122c85c78bbea05833d7c787cd184de568c5c45f Reviewed-on: https://chromium-review.googlesource.com/465606 Commit-Queue: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Corentin Wallez <cwallez@chromium.org>