Metal: Lazily grab the shader lib for provoking vertex helper Metal's ProvokingVertexHelper would query the display's default shader library in the constructor which is called during context creation. Lazily query the default shader library so that context creation is unblocked. Bug: chromium:1385510 Change-Id: I32ce3bdac7f92b7e098a69fa557aa8385eac7076 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4189016 Commit-Queue: Kenneth Russell <kbr@chromium.org> Reviewed-by: Jonah Ryan-Davis <jonahr@google.com> Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Kenneth Russell <kbr@chromium.org>