Revert "Don't mark all macros with double underscores as reserved." Fails a WebGL CTS test. BUG=angleproject:989 This reverts commit 942e36254a1e3537371c39f3f23d1ce12f4c87e8. Change-Id: I9f833366d5b69535ef74e358ac21efaccb1f1a3d Reviewed-on: https://chromium-review.googlesource.com/268751 Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>