sync: remove use of protobuf extensions in protocol to reduce static init overhead.

chrome/browser/sync/protocol/extension_setting_specifics.proto

Index: chrome/browser/sync/protocol/extension_setting_specifics.proto
diff --git a/chrome/browser/sync/protocol/extension_setting_specifics.proto b/chrome/browser/sync/protocol/extension_setting_specifics.proto
index ff9f029d4b0de9933162a30fb395f63bcc7859c7..d7e422870d54964b9511390b20fa1b97ecbf902d 100644
--- a/chrome/browser/sync/protocol/extension_setting_specifics.proto
+++ b/chrome/browser/sync/protocol/extension_setting_specifics.proto
@@ -14,8 +14,6 @@ option retain_unknown_fields = true;
 
 package sync_pb;
 
-import "sync.proto";
-
 // Properties of extension setting sync objects.
 message ExtensionSettingSpecifics {
   // Id of the extension the setting is for.
@@ -28,6 +26,3 @@ message ExtensionSettingSpecifics {
   optional string value = 3;
 }
 
-extend EntitySpecifics {
-  optional ExtensionSettingSpecifics extension_setting = 96159;
-}