This class uses no Impl-specific code. We can also do a similar
code relocation for the gl::Shader class, but in several steps
because it is a bit more complex.

BUG=angleproject:1159

Change-Id: I4d3ce3253df0a2bdee1d98e46cfd4b999d86be6e
Reviewed-on: https://chromium-review.googlesource.com/299874Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>