dawn_wire: Add Reserve/InjectDevice

Now that the wire does enough tracking to prevent a malicious client
from freeing a device before its child objects, and the device is no
longer a "special" object with regard to reference/release, it is
safe to support multiple devices on the wire. The simplest way to
use this in WebGPU (to fix createReadyRenderPipeline validation)
is to add a reserve/inject device API similar to the one we use for
swapchain textures.

Bug: dawn:565
Change-Id: Ie956aff528c5610c9ecc5c189dab2d22185cb572
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/37800
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Austin Eng <enga@chromium.org>

src/dawn_wire/client/Client.cpp

@@ -85,8 +85,13 @@ namespace dawn_wire { namespace client {
     }
 
     WGPUDevice Client::GetDevice() {
+        // This function is deprecated. The concept of a "default" device on the wire
+        // will be removed in favor of ReserveDevice/InjectDevice.
         if (mDevice == nullptr) {
-            mDevice = DeviceAllocator().New(this)->object.get();
+            ReservedDevice reservation = ReserveDevice();
+            mDevice = FromAPI(reservation.device);
+            ASSERT(reservation.id == 1);
+            ASSERT(reservation.generation == 0);
         }
         return reinterpret_cast<WGPUDeviceImpl*>(mDevice);
     }
@@ -103,6 +108,16 @@ namespace dawn_wire { namespace client {
         return result;
     }
 
+    ReservedDevice Client::ReserveDevice() {
+        auto* allocation = DeviceAllocator().New(this);
+
+        ReservedDevice result;
+        result.device = ToAPI(allocation->object.get());
+        result.id = allocation->object->id;
+        result.generation = allocation->generation;
+        return result;
+    }
+
     void Client::Disconnect() {
         mDisconnected = true;
         mSerializer = ChunkedCommandSerializer(NoopCommandSerializer::GetInstance());