Extensions: Store disable reasons in Sync

sync/protocol/extension_specifics.proto

Index: sync/protocol/extension_specifics.proto
diff --git a/sync/protocol/extension_specifics.proto b/sync/protocol/extension_specifics.proto
index 8e724a31916b367da696b8021c11feab393c0ba8..a854e013c968f974a54bec46c0df2a6eaf3656c9 100644
--- a/sync/protocol/extension_specifics.proto
+++ b/sync/protocol/extension_specifics.proto
@@ -48,5 +48,10 @@ message ExtensionSpecifics {
   // optional and may be absent. We need this for the time being because we need
   // to know if a user has not set an explicit preference.
   optional bool all_urls_enabled = 9;
+
+  // Bitmask of the set of reasons why the extension is disabled (see
+  // Extension::DisableReason). Only relevant when enabled == false. Note that
+  // old clients (<M45) won't set this, even when enabled is false.
+  optional int32 disable_reasons = 10;
 }