Lazily initialize MessageLoop for faster thread startup

base/threading/platform_thread.h

Index: base/threading/platform_thread.h
diff --git a/base/threading/platform_thread.h b/base/threading/platform_thread.h
index 28743145610c70fe14f8c8fdd1e377a9aa861528..0c495ec6464b078f5ba1722a96b934668caed2a9 100644
--- a/base/threading/platform_thread.h
+++ b/base/threading/platform_thread.h
@@ -89,6 +89,10 @@ class PlatformThreadHandle {
         id_(id) {
   }
 
+  PlatformThreadId id() const {
+    return id_;
+  }
+
   bool is_equal(const PlatformThreadHandle& other) const {
     return handle_ == other.handle_;
   }