Our angle_config.h was using "using namespace" in a header as a way to
import symbols to shim Chromium's base. This patch changes this to
import each function separately using "using".

This patch will allow our gpu_test_expectations to compile inside
Chromium on Linux.

BUG=angleproject:1127

Change-Id: I99a78ab9fddc11b81e3e5154d41cd8bbbb2ac28d
Reviewed-on: https://chromium-review.googlesource.com/294821Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>