PPAPI: Introduce concept of ResourceHosts "pinning" each other

base/memory/linked_ptr.h

Index: base/memory/linked_ptr.h
diff --git a/base/memory/linked_ptr.h b/base/memory/linked_ptr.h
index 80044add81c22cbd63f9d1d8fefa6ea077d3652b..251ba169e14785f70298700f4fb6d0f6ea4d1e0a 100644
--- a/base/memory/linked_ptr.h
+++ b/base/memory/linked_ptr.h
@@ -134,6 +134,8 @@ class linked_ptr {
   bool operator!=(linked_ptr<U> const& ptr) const {
     return value_ != ptr.get();
   }
+  // To allow use as a key in map or set.
+  bool operator<(linked_ptr<T> const& ptr) const { return value_ < ptr.get(); }
 
  private:
   template <typename U>