Fix the bug that closing a tab which owns a fullscreen Flash widget causes crash.

webkit/plugins/ppapi/ppapi_plugin_instance.cc

Index: webkit/plugins/ppapi/ppapi_plugin_instance.cc
diff --git a/webkit/plugins/ppapi/ppapi_plugin_instance.cc b/webkit/plugins/ppapi/ppapi_plugin_instance.cc
index 6df29eee00de3611a04dcc1ac5614035d078150e..3da10c702e3cc3782c58812954fe3aec008feb5d 100644
--- a/webkit/plugins/ppapi/ppapi_plugin_instance.cc
+++ b/webkit/plugins/ppapi/ppapi_plugin_instance.cc
@@ -260,6 +260,8 @@ PluginInstance::PluginInstance(
 }
 
 PluginInstance::~PluginInstance() {
+  DCHECK(!fullscreen_container_);
+
   // Free all the plugin objects. This will automatically clear the back-
   // pointer from the NPObject so WebKit can't call into the plugin any more.
   //