dawn_native: Add Instance and Adapters

New objects are introduced to control what happens before device
creation in dawn_native:
 - Instance: a connection from the application to dawn_native that is
used for dependency injection and to discover adapters.
 - Adapters: represents the possibility of device creation for a specific
(GPU, backend) pair.
 - BackendConnection: an internal object that standardizes the interface
between the frontend and backends.

The BackendConnection interface is implemented for the Null backend and
stubbed out in other backends. This allows this change to port the
ValidationTests to use the new Instance and Adapters concept and deal
with other backends later.

BUG=dawn:29

Change-Id: I19719a9342b4af091accc0c02fb6b9697eadde7b
Reviewed-on: https://dawn-review.googlesource.com/c/3500
Reviewed-by: Stephen White <senorblanco@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/dawn_native/DawnNative.cpp

@@ -14,6 +14,7 @@
 
 #include "dawn_native/DawnNative.h"
 #include "dawn_native/Device.h"
+#include "dawn_native/Instance.h"
 
 // Contains the entry-points into dawn_native
 
@@ -30,4 +31,50 @@ namespace dawn_native {
         return deviceBase->GetPCIInfo();
     }
 
+    // Adapter
+
+    Adapter::Adapter() = default;
+
+    Adapter::Adapter(AdapterBase* impl) : mImpl(impl) {
+    }
+
+    Adapter::~Adapter() {
+        mImpl = nullptr;
+    }
+
+    BackendType Adapter::GetBackendType() const {
+        return mImpl->GetBackendType();
+    }
+
+    const PCIInfo& Adapter::GetPCIInfo() const {
+        return mImpl->GetPCIInfo();
+    }
+
+    dawnDevice Adapter::CreateDevice() {
+        return reinterpret_cast<dawnDevice>(mImpl->CreateDevice());
+    }
+
+    // Instance
+
+    Instance::Instance() : mImpl(new InstanceBase()) {
+    }
+
+    Instance::~Instance() {
+        delete mImpl;
+        mImpl = nullptr;
+    }
+
+    void Instance::DiscoverDefaultAdapters() {
+        mImpl->DiscoverDefaultAdapters();
+    }
+
+    std::vector<Adapter> Instance::GetAdapters() const {
+        // Adapters are owned by mImpl so it is safe to return non RAII pointers to them
+        std::vector<Adapter> adapters;
+        for (const std::unique_ptr<AdapterBase>& adapter : mImpl->GetAdapters()) {
+            adapters.push_back({adapter.get()});
+        }
+        return adapters;
+    }
+
 }  // namespace dawn_native