Theme suppliers: Avoid all Image copying.

chrome/browser/themes/browser_theme_pack.h

Index: chrome/browser/themes/browser_theme_pack.h
diff --git a/chrome/browser/themes/browser_theme_pack.h b/chrome/browser/themes/browser_theme_pack.h
index 5d5d09269aa84b9ec32203dd32cdd54d8023f20f..e01f24b6a57b1fd2600ed4a390a7998d4cdead28 100644
--- a/chrome/browser/themes/browser_theme_pack.h
+++ b/chrome/browser/themes/browser_theme_pack.h
@@ -84,7 +84,7 @@ class BrowserThemePack : public CustomThemeSupplier {
   bool GetTint(int id, color_utils::HSL* hsl) const override;
   bool GetColor(int id, SkColor* color) const override;
   bool GetDisplayProperty(int id, int* result) const override;
-  gfx::Image GetImageNamed(int id) override;
+  const gfx::Image& GetImageNamed(int id) override;
   base::RefCountedMemory* GetRawData(int id, ui::ScaleFactor scale_factor)
       const override;
   bool HasCustomImage(int id) const override;
@@ -181,8 +181,7 @@ class BrowserThemePack : public CustomThemeSupplier {
 
   // Takes all images in |source| and puts them in |destination|, freeing any
   // image already in |destination| that |source| would overwrite.
-  void MergeImageCaches(const ImageCache& source,
-                        ImageCache* destination) const;
+  void MergeImageCaches(ImageCache source, ImageCache* destination) const;
 
   // Copies images from |source| to |destination| such that the lifetimes of
   // the images in |destination| are not affected by the lifetimes of the