Delete unused themes from disk on shutdown.

chrome/browser/browser_theme_provider.h

Index: chrome/browser/browser_theme_provider.h
diff --git a/chrome/browser/browser_theme_provider.h b/chrome/browser/browser_theme_provider.h
index f54c3ca8178f7a10dec278eb69fde2867f4a0a7d..af112859f2bd6e3e9717fed0c3c7d4d65b33ec27 100644
--- a/chrome/browser/browser_theme_provider.h
+++ b/chrome/browser/browser_theme_provider.h
@@ -336,6 +336,9 @@ class BrowserThemeProvider : public NonThreadSafe,
   // Frees generated images and clears the image cache.
   void ClearCaches();
 
+  // Remove preference values for themes that are no longer in use.
+  void RemoveUnusedThemes();
+
   // Encode image at image_cache_[id] as PNG and write to disk.
   void WriteImagesToDisk() const;