Add GetStoreSize() to ImageStore to know the actual db size.

components/enhanced_bookmarks/image_store.h

Index: components/enhanced_bookmarks/image_store.h
diff --git a/components/enhanced_bookmarks/image_store.h b/components/enhanced_bookmarks/image_store.h
index a7c7a15da1fcfb4af8200773b5b649955393c318..9d79506f861beabff07778aaf8f06ce4125cdd0e 100644
--- a/components/enhanced_bookmarks/image_store.h
+++ b/components/enhanced_bookmarks/image_store.h
@@ -52,6 +52,10 @@ class ImageStore {
   // Moves an image from one url to another.
   void ChangeImageURL(const GURL& from, const GURL& to);
 
+  // Returns the saved images storage size in bytes. If the storage doesn't
+  // exist yet or failed to read, returns -1.
+  virtual int64 GetStoreSizeInBytes() = 0;
+
  protected:
   base::SequenceChecker sequence_checker_;