Only __FILE__, __LINE__, __VERSION__ and GL_ES are reserved but it is
still not recommended to use a name with double underscores because it may
be used by the "underlying software layers".

Updated the tests to reflect that it is OK to define macros with double
underscores but it is not valid to make assumptions about their values.

Fixes:
dEQP-GLES2.functional.shaders.preprocessor.basic.identifier_with_double_underscore_vertex
dEQP-GLES2.functional.shaders.preprocessor.basic.identifier_with_double_underscore_fragment

BUG=angleproject:898

Change-Id: I77054d04c9935eedcdbb7304dc0c3b60b53994f9
Reviewed-on: https://chromium-review.googlesource.com/268434Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>