De-templatize ResourceManagerBase. Bug: skia:7647 Change-Id: I568cb63118c944c7eca7efcd2445718dc7fda350 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2404745 Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>