Allow double underscore in macro names Double underscore is allowed according to GLSL ES 3.10, and based on Khronos discussions the intent is that this should also apply to older specs. The dEQP tests also check this, and WebGL tests that check the opposite were recently removed. The error is changed into a warning. BUG=angleproject:989 TEST=angle_unittests dEQP-GLES3.functional.shaders.preprocessor.* (2 tests start passing) dEQP-GLES2.functional.shaders.preprocessor.* (2 tests start passing) Change-Id: I582c01b4adc8fc416354351e02b776f2cc602408 Reviewed-on: https://chromium-review.googlesource.com/300965 Tested-by: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Zhenyao Mo <zmo@chromium.org> Tryjob-Request: Olli Etuaho <oetuaho@nvidia.com>