Move the shared validation logic to validationES.cpp.

BUG=angle:571

Change-Id: I2fb06a012703c647de2176914238b48665145c6f
Reviewed-on: https://chromium-review.googlesource.com/200073Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>