Add ContainsKey() and ContainsValue() and use them where appropriate.

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 fcbe5882a20ab6d64e5bedff2847678e0efa60c8..50e93415699cb395313b809a95b697016698f16a 100644
--- a/third_party/base/stl_util.h
+++ b/third_party/base/stl_util.h
@@ -22,6 +22,21 @@ inline const T* vector_as_array(const std::vector<T>* v) {
   return v->empty() ? nullptr : &*v->begin();
 }
 
+// Test to see if a set, map, hash_set or hash_map contains a particular key.
+// Returns true if the key is in the collection.
+template <typename Collection, typename Key>
+bool ContainsKey(const Collection& collection, const Key& key) {
+  return collection.find(key) != collection.end();
+}
+
+// Test to see if a collection like a vector contains a particular value.
+// Returns true if the value is in the collection.
+template <typename Collection, typename Value>
+bool ContainsValue(const Collection& collection, const Value& value) {
+  return std::find(collection.begin(), collection.end(), value) !=
+         collection.end();
+}
+
 }  // namespace pdfium
 
 #endif  // PDFIUM_THIRD_PARTY_BASE_STL_UTIL_H_