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/258900Reviewed-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>