misc/pprof: don't look for browser in current directory.

Taken from upstream pprof.

Fixes #4564.

R=golang-dev, minux.ma
CC=golang-dev
https://golang.org/cl/6952045

misc/pprof

@@ -726,12 +726,10 @@ sub RunWeb {
     "firefox",
   );
   foreach my $b (@alt) {
-    if (-f $b) {
     if (system($b, $fname) == 0) {
       return;
     }
   }
-  }
 
   print STDERR "Could not load web browser.\n";
 }