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

This fixes a syntax error in plot-timer-events script.

Before:
  $ out/x64.release/d8 --prof --log-timer-events script.js
  $ tools/plot-timer-events v8.log
  tools/plot-timer-events: 75: tools/plot-timer-events: options+=1343: not found
  --distortion=
  Cmdline args: [options] [log-file-name]
  Default log file name is "v8.log".
    ...

After:
  $ out/x64.release/d8 --prof --log-timer-events script.js
  $ tools/plot-timer-events v8.log
  $ ls timer-events.png
  timer-events.png

BUG=

Review-Url: https://codereview.chromium.org/1961873002
Cr-Commit-Position: refs/heads/master@{#36434}

AUTHORS

@@ -63,6 +63,7 @@ Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
 Geoffrey Garside <ggarside@gmail.com>
 Han Choongwoo <cwhan.tunz@gmail.com>
 Hirofumi Mako <mkhrfm@gmail.com>
+Honggyu Kim <nexus226@gmail.com>
 Ioseb Dzmanashvili <ioseb.dzmanashvili@gmail.com>
 Isiah Meadows <impinball@gmail.com>
 Jan de Mooij <jandemooij@gmail.com>

tools/plot-timer-events

@@ -70,10 +70,9 @@ if test "$contains" -eq 0; then
   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 |