Deprecate TextureDescriptor::arrayLayerCount -> size.depth

This updates CL:

 - Adds a deprecation warning to use size.depth instead of
arrayLayerCount.
 - Changes all tests and samples to use size.depth.
 - Adds deprecation tests for the change.

In particular the state tracking in TextureBase isn't changed yet
because it requires non-trivial changes in the backends. It will be done
in a follow-up CL.

Bug:dawn:22

Change-Id: Ic02dfb5baaba8d5b06cd339ce988e9b1d16cb5e9
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/23101
Reviewed-by: Jiawei Shao <jiawei.shao@intel.com>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

examples/CppHelloTriangle.cpp

@@ -56,7 +56,6 @@ void initTextures() {
     descriptor.size.width = 1024;
     descriptor.size.height = 1024;
     descriptor.size.depth = 1;
-    descriptor.arrayLayerCount = 1;
     descriptor.sampleCount = 1;
     descriptor.format = wgpu::TextureFormat::RGBA8Unorm;
     descriptor.mipLevelCount = 1;