Ensure that PP_Instance values are unique within a plugin process in addition...

content/common/child_process.h

Index: content/common/child_process.h
===================================================================
--- content/common/child_process.h	(revision 77663)
+++ content/common/child_process.h	(working copy)
@@ -18,11 +18,15 @@
 class ChildProcess {
  public:
   // Child processes should have an object that derives from this class.
+  // Normally you would immediately call set_main_thread after construction.
   ChildProcess();
   virtual ~ChildProcess();
 
-  // Getter for the child process' main thread.
+  // May be NULL if the main thread hasn't been set explicitly.
   ChildThread* main_thread();
+
+  // Sets the object associated with the main thread of this process.
+  // Takes ownership of the pointer.
   void set_main_thread(ChildThread* thread);
 
   MessageLoop* io_message_loop() { return io_thread_.message_loop(); }