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

chrome/browser/sync/protocol/theme_specifics.proto

Index: chrome/browser/sync/protocol/theme_specifics.proto
diff --git a/chrome/browser/sync/protocol/theme_specifics.proto b/chrome/browser/sync/protocol/theme_specifics.proto
index eb864ff0da16b855efd7f847278130f087986fde..af8dbf050da929d924ff66789af938a0a400b7e6 100644
--- a/chrome/browser/sync/protocol/theme_specifics.proto
+++ b/chrome/browser/sync/protocol/theme_specifics.proto
@@ -14,8 +14,6 @@ option retain_unknown_fields = true;
 
 package sync_pb;
 
-import "sync.proto";
-
 // Properties of theme sync objects.
 message ThemeSpecifics {
   // If set, we're using a custom theme and all custom_* fields should be
@@ -39,6 +37,3 @@ message ThemeSpecifics {
   optional string custom_theme_update_url = 5;
 }
 
-extend EntitySpecifics {
-  optional ThemeSpecifics theme = 41210;
-}