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

chrome/browser/sync/protocol/session_specifics.proto

Index: chrome/browser/sync/protocol/session_specifics.proto
diff --git a/chrome/browser/sync/protocol/session_specifics.proto b/chrome/browser/sync/protocol/session_specifics.proto
index a4a94b3dba027c1ef27cb2c188b18ee6af685845..5b332db83dcae8f8f6d795950e4e4ed3763fc5a6 100644
--- a/chrome/browser/sync/protocol/session_specifics.proto
+++ b/chrome/browser/sync/protocol/session_specifics.proto
@@ -14,8 +14,6 @@ option retain_unknown_fields = true;
 
 package sync_pb;
 
-import "sync.proto";
-
 message SessionSpecifics {
   // Unique id for the client.
   optional string session_tag = 1;
@@ -111,6 +109,3 @@ message TabNavigation {
   optional PageTransitionQualifier navigation_qualifier = 7;
 }
 
-extend EntitySpecifics {
-  optional SessionSpecifics session = 50119;
-}