Raise the loading priority of in-viewport images.

LayoutTests/http/tests/misc/resources/image-slow-out-of-viewport.pl

 #!/usr/bin/perl -w
 
 # flush the buffers after each print
 select (STDOUT);
 $| = 1;
 
 print "Content-Type: image/jpeg\r\n";
 print "Expires: Thu, 01 Dec 2003 16:00:00 GMT\r\n";
 print "Cache-Control: no-store, no-cache, must-revalidate\r\n";
 print "Pragma: no-cache\r\n";
 print "\r\n";
 
 sleep 2;
 
 open(FILE, "compass.jpg");
 while (<FILE>) {
   print $_;
 }
 close(FILE);