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 737449a2feb807a0456100fee4f9a3839197caca..05f82b13ee589324f62d2d8b152fda3d86bb48c6 100644
--- a/mojo/common/handle_watcher.cc
+++ b/mojo/common/handle_watcher.cc
@@ -458,6 +458,8 @@ 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();

[viettrungluu, 2014/12/08 18:10:01]

I assume that this is because we need the dtor of the old state to run before
the ctor for the new state (for at least one of
Same/SecondaryThreadWatchingState).

This is pretty unintuitive though, and mostly speaks to the badness of doing
stuff in dtors.

[qsr, 2014/12/09 10:02:23]

 The destructor remove the handler, and neither of those can watch the same
handle multiple times.

   if (MessagePumpMojo::IsCurrent()) {
     state_.reset(new SameThreadWatchingState(
         this, handle, handle_signals, deadline, callback));