Add a helper ShaderVariable method to facilitate member rename Chromium uses ShaderVariable::structName directly, which makes it impossible to rename this variable. Bug: angleproject:3580 Change-Id: I328015b0e7f26d22d689451bac0d7a48999bef2e Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2637419 Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>