tools: Fix a syntax error in plot-timer-events

tools/plot-timer-events

 #!/bin/sh
 
 # find the name of the log file to process, it must not start with a dash.
 log_file="v8.log"
 for arg in "$@"
 do
   if ! expr "X${arg}" : "^X-" > /dev/null; then
     log_file=${arg}
   fi
 done
@@ skipping 52 matching lines @@
        --log-internal-timer-events -e "$calibration_script" > /dev/null
   t_2_start=`grep "timer-event-start,\"V8.Execute\"" $calibration_log  \
        | tail -n1 | awk -F, '{print $3}'`
   t_2_end=`grep "timer-event-end,\"V8.Execute\"" $calibration_log  \
        | tail -n1 | awk -F, '{print $3}'`
   n_2=`grep "timer-event\|tick" $calibration_log  | wc -l`
 
   rm $calibration_log
 
   # Overhead in picoseconds.
-  options=--distortion=
-  options+=`echo "1000*(($t_1_end - $t_1_start) - ($t_2_end - $t_2_start)) \
+  distortion=`echo "1000*(($t_1_end - $t_1_start) - ($t_2_end - $t_2_start)) \
               / ($n_1 - $n_2)" | bc`
-  echo $options
+  options="--distortion=$distortion"
 fi
 
 cat $log_file |
     $d8_exec $tools_path/csvparser.js $tools_path/splaytree.js \
     $tools_path/codemap.js $tools_path/profile.js $tools_path/profile_view.js \
     $tools_path/logreader.js $tools_path/tickprocessor.js \
     $tools_path/profviz/composer.js $tools_path/profviz/stdio.js \
     -- $@ $options 2>/dev/null > timer-events.plot
 
 success=$?
 if test $success -ne 0; then
     cat timer-events.plot
 else
     cat timer-events.plot | gnuplot > timer-events.png
 fi
 
 rm -f timer-events.plot