Change ShaderLang APIs from c style to c++ style. BUG=angle:816 TEST=gpu_unittests,angle_unittests,webgl_conformance Change-Id: I0b46c11f6055a82511bb946a6dc491360835526e Reviewed-on: https://chromium-review.googlesource.com/226410 Reviewed-by: Zhenyao Mo <zmo@chromium.org> Tested-by: Zhenyao Mo <zmo@chromium.org>