With resources being mostly deleted by the Renderer along with a
serial number, the retain move semantics weren't very useful.

Refactoring change only.

BUG=angleproject:2200

Change-Id: I7b72b1decfa7604cdd767e7d9b5213b9383eb240
Reviewed-on: https://chromium-review.googlesource.com/742369Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>