This replaces the dual-use of sh::Attribute, which can be a bit
confusing to people expecting a literal output variable.

Currently not used in Chromium, so should be safe to land.

BUG=angleproject:1146

Change-Id: I436f2bc9dc4ddc3709369cb2baa344c6b13a21a2
Reviewed-on: https://chromium-review.googlesource.com/296683Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>