Move WorkerTask functionality into RunLoop core

As a byproduct, this makes FileCache get requests properly
cancelable.

platform/default/sqlite_cache.cpp

@@ -126,12 +126,12 @@ void SQLiteCache::Impl::createSchema() {
     }
 }
 
-void SQLiteCache::get(const Resource &resource, Callback callback) {
+std::unique_ptr<WorkRequest> SQLiteCache::get(const Resource &resource, Callback callback) {
     // Can be called from any thread, but most likely from the file source thread.
     // Will try to load the URL from the SQLite database and call the callback when done.
     // Note that the callback is probably going to invoked from another thread, so the caller
     // must make sure that it can run in that thread.
-    thread->invokeWithResult(&Impl::get, std::move(callback), resource);
+    return thread->invokeWithResult(&Impl::get, std::move(callback), resource);
 }
 
 std::unique_ptr<Response> SQLiteCache::Impl::get(const Resource &resource) {