resources: Prevent including the same resource in multiple pack files.

ui/base/resource/data_pack.h

Index: ui/base/resource/data_pack.h
diff --git a/ui/base/resource/data_pack.h b/ui/base/resource/data_pack.h
index 1bf87365d93dfa7970ba13796cf2595c7adb3349..1ca3f101240786bec2d07e15bde0772af3430277 100644
--- a/ui/base/resource/data_pack.h
+++ b/ui/base/resource/data_pack.h
@@ -15,6 +15,7 @@
 #include "base/files/file.h"
 #include "base/files/memory_mapped_file.h"
 #include "base/memory/scoped_ptr.h"
+#include "base/memory/scoped_vector.h"
 #include "base/strings/string_piece.h"
 #include "ui/base/resource/resource_handle.h"
 #include "ui/base/ui_base_export.h"
@@ -59,6 +60,12 @@ class UI_BASE_EXPORT DataPack : public ResourceHandle {
   TextEncodingType GetTextEncodingType() const override;
   ui::ScaleFactor GetScaleFactor() const override;
 
+#if DCHECK_IS_ON()
+  // Checks to see if any resource in this DataPack already exists in the list
+  // of resources.
+  void CheckForDuplicateResources(const ScopedVector<ResourceHandle>& packs);
+#endif
+
  private:
   // Does the actual loading of a pack file. Called by Load and LoadFromFile.
   bool LoadImpl();