Implement WaitableEvent and WorkerTaskPool for multi-threaded tasks

This patch adds the basic implementation of WaitableEvent and
WorkerTaskPool for multi-threaded tasks in Dawn (for example, the
multi-threaded implementation of CreateReady*Pipeline()).

BUG=dawn:529
TEST=dawn_unittests

Change-Id: Ibf84348f4c0f0d26badc19ae94cd536cef89d084
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/36360
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jiawei Shao <jiawei.shao@intel.com>

src/include/dawn_platform/DawnPlatform.h

@@ -19,6 +19,7 @@
 
 #include <cstddef>
 #include <cstdint>
+#include <memory>
 
 #include <dawn/webgpu.h>
 
@@ -60,6 +61,24 @@ namespace dawn_platform {
         CachingInterface& operator=(const CachingInterface&) = delete;
     };
 
+    class DAWN_PLATFORM_EXPORT WaitableEvent {
+      public:
+        WaitableEvent() = default;
+        virtual ~WaitableEvent() = default;
+        virtual void Wait() = 0;        // Wait for completion
+        virtual bool IsComplete() = 0;  // Non-blocking check if the event is complete
+    };
+
+    using PostWorkerTaskCallback = void (*)(void* userdata);
+
+    class DAWN_PLATFORM_EXPORT WorkerTaskPool {
+      public:
+        WorkerTaskPool() = default;
+        virtual ~WorkerTaskPool() = default;
+        virtual std::unique_ptr<WaitableEvent> PostWorkerTask(PostWorkerTaskCallback,
+                                                              void* userdata) = 0;
+    };
+
     class DAWN_PLATFORM_EXPORT Platform {
       public:
         Platform();
@@ -85,6 +104,7 @@ namespace dawn_platform {
         // device which uses it to persistently cache objects.
         virtual CachingInterface* GetCachingInterface(const void* fingerprint,
                                                       size_t fingerprintSize);
+        virtual std::unique_ptr<WorkerTaskPool> CreateWorkerTaskPool();
 
       private:
         Platform(const Platform&) = delete;