[subresource_filter] Index activation rules separately.

components/subresource_filter/core/common/flat/rules.fbs

Index: components/subresource_filter/core/common/flat/rules.fbs
diff --git a/components/subresource_filter/core/common/flat/rules.fbs b/components/subresource_filter/core/common/flat/rules.fbs
index d50de14149bc3ff96971cb5e1c0f153b89c4683c..f60253558b5d7e1859fb6edb1ce7496d49457f2c 100644
--- a/components/subresource_filter/core/common/flat/rules.fbs
+++ b/components/subresource_filter/core/common/flat/rules.fbs
@@ -94,8 +94,11 @@ table IndexedRuleset {
   // The index of all blacklist URL rules.
   blacklist_index : UrlPatternIndex;
 
-  // The index of all whitelist URL rules.
+  // The index of all whitelist URL rules, except pure activation rules.
   whitelist_index : UrlPatternIndex;
+
+  // The index of all whitelist URL rules with activation options.
+  activation_index : UrlPatternIndex;
 }
 
 root_type IndexedRuleset;