Make TextureDescriptor match WebGPU IDL

This patch updates the definition of TextureDescriptor to make it match
WebGPU IDL:
1. Rename 'arrayLayer' to 'arraySize'
2. Add the missing member "sampleCount" and check that currently
   sampleCount can only be 1.

BUG=dawn:56
TEST=dawn_unittests

Change-Id: I642186529f045865ae344cb5545ac80e14445c59
Reviewed-on: https://dawn-review.googlesource.com/c/3180
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
diff --git a/examples/CppHelloTriangle.cpp b/examples/CppHelloTriangle.cpp
index f0f6794..44b2621 100644
--- a/examples/CppHelloTriangle.cpp
+++ b/examples/CppHelloTriangle.cpp
@@ -54,7 +54,8 @@
     descriptor.size.width = 1024;
     descriptor.size.height = 1024;
     descriptor.size.depth = 1;
-    descriptor.arrayLayer = 1;
+    descriptor.arraySize = 1;
+    descriptor.sampleCount = 1;
     descriptor.format = dawn::TextureFormat::R8G8B8A8Unorm;
     descriptor.levelCount = 1;
     descriptor.usage = dawn::TextureUsageBit::TransferDst | dawn::TextureUsageBit::Sampled;