Introduce pdfium::FakeUniquePtr for keys to sets of unique ptrs.

third_party/base/stl_util.h

Index: third_party/base/stl_util.h
diff --git a/third_party/base/stl_util.h b/third_party/base/stl_util.h
index 2d1846724c3094161ce910986fd13be10c7b9f62..ccf3c09073abd9e213addb85bd7ad333c29196e3 100644
--- a/third_party/base/stl_util.h
+++ b/third_party/base/stl_util.h
@@ -27,6 +27,15 @@ bool ContainsValue(const Collection& collection, const Value& value) {
          collection.end();
 }
 
+// Means of generating a key for searching STL collections of std::unique_ptr
+// that avoids the side effect of deleting the pointer.
+template <class T>
+class FakeUniquePtr : public std::unique_ptr<T> {
+ public:
+  using std::unique_ptr<T>::unique_ptr;
+  ~FakeUniquePtr() { std::unique_ptr<T>::release(); }
+};
+
 // Convenience routine for "int-fected" code, so that the stl collection
 // size_t size() method return values will be checked.
 template <typename ResultType, typename Collection>