Downgrade Leaked Rewrite Drivers message to kInfo

Closes #1039, downgrading to kInfo as discussed.

#1039

Author: crowell

net/instaweb/rewriter/server_context.cc

@@ -326,7 +326,7 @@ ServerContext::~ServerContext() {
   // We scan for "leaked_rewrite_drivers" in apache/install/Makefile.tests
   if (!active_rewrite_drivers_.empty()) {
     message_handler_->Message(
-        kError, "ServerContext: %d leaked_rewrite_drivers on destruction",
+        kInfo, "ServerContext: %d leaked_rewrite_drivers on destruction",
         static_cast<int>(active_rewrite_drivers_.size()));
 #ifndef NDEBUG
     for (RewriteDriverSet::iterator p = active_rewrite_drivers_.begin(),