Add support for creating non-WebGL ES 3 contexts.
Replaces "web gl version" in context creation attributes with
"context type".
Adds an example use-case, a GL test that uses ES 3.0 context.
Makes it possible to use ES 3.0 functions exposed by command buffer.
Previously the ES 3.0 functions were removed by the linker.
Needed, because up-coming features might have ES 3.0 related code,
and that code needs to be tested. EXT_blend_func_extended is an
example of such a feature.
BUG=506765
Review URL: https://codereview.chromium.org/1325433003
Cr-Commit-Position: refs/heads/master@{#350319}