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>

src/tests/unittests/validation/TextureSubresourceTests.cpp

@@ -28,8 +28,7 @@ namespace {
                                     wgpu::TextureUsage usage) {
             wgpu::TextureDescriptor texDesc;
             texDesc.dimension = wgpu::TextureDimension::e2D;
-            texDesc.size = {kSize, kSize, 1};
-            texDesc.arrayLayerCount = arrayLayerCount;
+            texDesc.size = {kSize, kSize, arrayLayerCount};
             texDesc.sampleCount = 1;
             texDesc.mipLevelCount = mipLevelCount;
             texDesc.usage = usage;