Add an ImplFactory base class. This class helps make mocking easier for unit tests. Also update the back-end generator script. BUG=angleproject:942 Change-Id: Ib0cee2b40c3a0faaac32b22c986b824b7b2dddde Reviewed-on: https://chromium-review.googlesource.com/258900 Reviewed-by: Brandon Jones <bajones@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>