Introduce a 'DeferredShader' utility class. Other classes can use
DeferredShader to delay loading shader resources until we render
with them, which saves on loading time as well as memory, if we
aren't using particular shaders.

BUG=angleproject:1014

Change-Id: Ic6767c3c422a7fedbf23cce5d0c9d822aaf2e652
Reviewed-on: https://chromium-review.googlesource.com/275778Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>