Support requesting subsets of host permissions using the permissions API.

chrome/common/extensions/permissions/permission_set.h

Index: chrome/common/extensions/permissions/permission_set.h
diff --git a/chrome/common/extensions/permissions/permission_set.h b/chrome/common/extensions/permissions/permission_set.h
index a2e3d10b24d15c0479b73f1addd079b4b0ea3afa..d242abde1bab442547a69139696e63fcdb0385a7 100644
--- a/chrome/common/extensions/permissions/permission_set.h
+++ b/chrome/common/extensions/permissions/permission_set.h
@@ -67,7 +67,9 @@ class PermissionSet
 
   bool operator==(const PermissionSet& rhs) const;
 
-  // Returns true if |set| is a subset of this.
+  // Returns true if every API or host permission available to |set| is also
+  // available to this. In other words, if the API permissions of |set| are a
+  // subset of this, and the host permissions in this encompass those in |set|.
   bool Contains(const PermissionSet& set) const;
 
   // Gets the API permissions in this set as a set of strings.