webgpu: Add the dedicated WebGPU task source (#39020)

According to the WebGPU specification there are the dedicated task
source
which is used to queue a global task for a GPUDevice on content
timeline.
https://gpuweb.github.io/gpuweb/#-webgpu-task-source

Tasks on content timeline:
- to fire "uncaptureevent" event
- to resolve GPUDevice.lost promise

Also fixed the "isTrusted" attribute status (false -> true) of the
"uncaptureevent" event by using non JS version of event dispatching.

Testing: No changes in WebGPU CTS expectations
- webgpu:api,operation,uncapturederror:*
- webgpu:api,operation,device,lost:*
- webgpu:api,validation,state,device_lost,destroy:*

Signed-off-by: Andrei Volykhin <volykhin.andrei@huawei.com>
Co-authored-by: Andrei Volykhin <volykhin.andrei@huawei.com>

components/script/dom/webgpu/gpuuncapturederrorevent.rs

@@ -62,10 +62,6 @@ impl GPUUncapturedErrorEvent {
         );
         ev
     }
-
-    pub(crate) fn event(&self) -> &Event {
-        &self.event
-    }
 }
 
 impl GPUUncapturedErrorEventMethods<crate::DomTypeHolder> for GPUUncapturedErrorEvent {