Remove CompilerImpl and merge code to gl::Compiler. 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/299874 Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>