Update duration when demuxed packets exceed known duration.

third_party/WebKit/LayoutTests/http/tests/resources/load-and-stall.cgi

-#!/usr/bin/perl -wT
-
-use CGI;
-use File::stat;
-use Time::HiRes;
-
-$query = new CGI;
-$name = $query->param('name');
-$stallAt = $query->param('stallAt');
-$stallFor = $query->param('stallFor');
-$mimeType = $query->param('mimeType');
-
-my $filesize = stat($name)->size;
-print "Content-type: " . $mimeType . "\n"; 
-print "Content-Length: " . $filesize . "\n\n";
-
-open FILE, $name or die;
-binmode FILE;
-$total = 0;
-my ($buf, $data, $n);
-while (($n = read FILE, $data, 1024) != 0) {
-    $total += $n;
-    if ($total > $stallAt) {
-        if (defined $stallFor) {
-            Time::HiRes::sleep($stallFor)
-        }
-        last;
-    }
-    print $data;
-}
-close(FILE);