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>