This reduces the size of the RenderPass desc from 64 to 12 bytes.

Bug: angleproject:2522
Change-Id: Iff2df87ba65be0bd976bba81c76c285cb0fa1ceb
Reviewed-on: https://chromium-review.googlesource.com/c/1308459Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>