In preparation for some clear-related optimizations.

Bug: angleproject:4836
Change-Id: I32ceb2b8a690ed13dac81e1ffe073436d6e24421
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2453464
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>