Updates DawnInstanceDescriptor to pass in the Platform.

Notes:
- Separates ChainedStruct to be reusable without cpp header. (Also
  updates native structs to directly use it.)
- Manually implements the descriptor in DawnNative.
- Reworks ChainUtils with mapping from struct to STypes.
- Updates the tests to use either SetPlatformForTesting which is still
  required because DawnTest uses a "global" instance for all tests and
  some tests require setting (and cleaning up) a test specific platform.

Bug: dawn:1374
Change-Id: I078c78f22c5137030cf3cf0e8358fe4373ee9c6c
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/132268
Reviewed-by: Austin Eng <enga@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Loko Kung <lokokung@google.com>

include/dawn/native/DawnNative.h

@@ -21,6 +21,7 @@
 #include "dawn/dawn_proc_table.h"
 #include "dawn/native/dawn_native_export.h"
 #include "dawn/webgpu.h"
+#include "dawn/webgpu_cpp_chained_struct.h"
 
 namespace dawn::platform {
 class Platform;
@@ -128,6 +129,20 @@ struct DAWN_NATIVE_EXPORT AdapterDiscoveryOptionsBase {
 
 enum BackendValidationLevel { Full, Partial, Disabled };
 
+// Can be chained in InstanceDescriptor
+struct DAWN_NATIVE_EXPORT DawnInstanceDescriptor : wgpu::ChainedStruct {
+    DawnInstanceDescriptor();
+    static constexpr size_t kFirstMemberAlignment =
+        wgpu::detail::ConstexprMax(alignof(wgpu::ChainedStruct), alignof(uint32_t));
+    alignas(kFirstMemberAlignment) uint32_t additionalRuntimeSearchPathsCount = 0;
+    const char* const* additionalRuntimeSearchPaths;
+    dawn::platform::Platform* platform = nullptr;
+
+    // Equality operators, mostly for testing. Note that this tests
+    // strict pointer-pointer equality if the struct contains member pointers.
+    bool operator==(const DawnInstanceDescriptor& rhs) const;
+};
+
 // Represents a connection to dawn_native and is used for dependency injection, discovering
 // system adapters and injecting custom adapters (like a Swiftshader Vulkan adapter).
 //
@@ -262,6 +277,12 @@ DAWN_NATIVE_EXPORT bool BindGroupLayoutBindingsEqualForTesting(WGPUBindGroupLayo
 
 }  // namespace dawn::native
 
+// Alias the DawnInstanceDescriptor up to wgpu.
+// TODO(dawn:1374) Remove this aliasing once the usages are updated.
+namespace wgpu {
+using dawn::native::DawnInstanceDescriptor;
+}  // namespace wgpu
+
 // TODO(dawn:824): Remove once the deprecation period is passed.
 namespace dawn_native = dawn::native;