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

chrome/browser/sync/protocol/app_setting_specifics.proto

Index: chrome/browser/sync/protocol/app_setting_specifics.proto
diff --git a/chrome/browser/sync/protocol/app_setting_specifics.proto b/chrome/browser/sync/protocol/app_setting_specifics.proto
index 0d9e25d685bd104e49b552c494bd86a41d4fb94e..62a8c7a67bf84921ce519a53654aa8be09265b79 100644
--- a/chrome/browser/sync/protocol/app_setting_specifics.proto
+++ b/chrome/browser/sync/protocol/app_setting_specifics.proto
@@ -16,7 +16,6 @@ option retain_unknown_fields = true;
 
 package sync_pb;
 
-import "sync.proto";
 import "extension_setting_specifics.proto";
 
 // Properties of app setting sync objects; just an extension setting.
@@ -24,6 +23,3 @@ message AppSettingSpecifics {
   optional ExtensionSettingSpecifics extension_setting = 1;
 }
 
-extend EntitySpecifics {
-  optional AppSettingSpecifics app_setting = 103656;
-}