Fix handle_watcher when start is called twice on the same handle.

mojo/common/handle_watcher.cc

Index: mojo/common/handle_watcher.cc
diff --git a/mojo/common/handle_watcher.cc b/mojo/common/handle_watcher.cc
index 7819779911a9827af262397bd94952a0ff3dd126..79cdcdb616b7e8cc5c1b69603b945b52f06ced65 100644
--- a/mojo/common/handle_watcher.cc
+++ b/mojo/common/handle_watcher.cc
@@ -459,6 +459,8 @@ void HandleWatcher::Start(const Handle& handle,
   DCHECK(handle.is_valid());
   DCHECK_NE(MOJO_HANDLE_SIGNAL_NONE, handle_signals);
 
+  // Need to clear the state before creating a new one.
+  state_.reset();
   if (MessagePumpMojo::IsCurrent()) {
     state_.reset(new SameThreadWatchingState(
         this, handle, handle_signals, deadline, callback));