Extension Storage API: expose storage quota information to extensions, via:

chrome/browser/extensions/settings/settings_storage.h

Index: chrome/browser/extensions/settings/settings_storage.h
diff --git a/chrome/browser/extensions/settings/settings_storage.h b/chrome/browser/extensions/settings/settings_storage.h
index 4c5c67403ff1bef61b8c57a6512b16d48aa85ac4..51cc6032b6b568b95e3a6aaefee2e9cb9bf3dba4 100644
--- a/chrome/browser/extensions/settings/settings_storage.h
+++ b/chrome/browser/extensions/settings/settings_storage.h
@@ -100,6 +100,15 @@ class SettingsStorage {
 
   virtual ~SettingsStorage() {}
 
+  // Gets the amount of space being used by a single value, in bytes.
+  virtual size_t GetBytesInUse(const std::string& key) = 0;
+
+  // Gets the total amount of space being used by multiple values, in bytes.
+  virtual size_t GetBytesInUse(const std::vector<std::string>& keys) = 0;
+
+  // Gets the total amount of space being used by this storage area, in bytes.
+  virtual size_t GetBytesInUse() = 0;
+
   // Gets a single value from storage.
   virtual ReadResult Get(const std::string& key) = 0;