Revert making HandleWatcher block until no longer waiting on pipe (r285266).

base/threading/thread_restrictions.h

Index: base/threading/thread_restrictions.h
diff --git a/base/threading/thread_restrictions.h b/base/threading/thread_restrictions.h
index b124c9ff6ab2d1fc0353595d2a3b9a59a5dcf390..fd03bcdfbef2199d76a5ce3b27d3d365543c044c 100644
--- a/base/threading/thread_restrictions.h
+++ b/base/threading/thread_restrictions.h
@@ -55,11 +55,6 @@ class InFlightIO;
 namespace gles2 {
 class CommandBufferClientImpl;
 }
-namespace mojo {
-namespace common {
-class WatcherThreadManager;
-}
-}
 namespace net {
 class NetworkChangeNotifierMac;
 namespace internal {
@@ -192,7 +187,6 @@ class BASE_EXPORT ThreadRestrictions {
   friend class ::ScopedAllowWaitForLegacyWebViewApi;
   friend class cc::CompletionEvent;
   friend class cc::TaskGraphRunner;
-  friend class mojo::common::WatcherThreadManager;
   friend class remoting::AutoThread;
   friend class ui::WindowResizeHelperMac;
   friend class MessagePumpDefault;