Simplify has function for level/layer of imageView and add
asserts to make sure that there won't be hash collisions.

Bug: angleproject:4651
Change-Id: I8ab86a4f3d7aa668ad2c08e61bd5fd57744fcd12
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2257265Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Tobin Ehlis <tobine@google.com>