Remove unused thread.h include in trace_event_impl.h and fix IWUU.

components/leveldb/leveldb_mojo_proxy.h

Index: components/leveldb/leveldb_mojo_proxy.h
diff --git a/components/leveldb/leveldb_mojo_proxy.h b/components/leveldb/leveldb_mojo_proxy.h
index 881e8b7e0dacdaf3234554bb7b3bcb00820fda05..2eef1a41d1db0bd2f5a04886e1195cde7b97ef7f 100644
--- a/components/leveldb/leveldb_mojo_proxy.h
+++ b/components/leveldb/leveldb_mojo_proxy.h
@@ -15,9 +15,12 @@
 #include "base/files/file.h"
 #include "base/memory/ref_counted.h"
 #include "base/synchronization/waitable_event.h"
-#include "base/threading/thread.h"
 #include "components/filesystem/public/interfaces/directory.mojom.h"
 
+namespace base {
+class SingleThreadTaskRunner;
+}
+
 namespace leveldb {
 
 // A proxy for thread safe access to Mojo objects from multiple threads.