This will make mocking easier for unit testing ResourceManager.

BUG=angleproject:942

Change-Id: I24b9d0be550991845225141d6e7d5630383a8718
Reviewed-on: https://chromium-review.googlesource.com/258901Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>