Fix crasher in the pdf tear down code path.

pdf/instance.cc

Index: pdf/instance.cc
diff --git a/pdf/instance.cc b/pdf/instance.cc
index d779089f0067f5d23829eede408644640bcabbda..9cb78294b090980824760b2b3a51dddeb026c48e 100644
--- a/pdf/instance.cc
+++ b/pdf/instance.cc
@@ -307,6 +307,10 @@ Instance::Instance(PP_Instance instance)
 }
 
 Instance::~Instance() {
+  if (timer_pending_) {
+    timer_factory_.CancelAll();
+    timer_pending_ = false;
+  }
   // The engine may try to access this instance during its destruction.
   // Make sure this happens early while the instance is still intact.
   engine_.reset();