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/300965Tested-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>