Don't fail taskkill on chrome processes alive

It happens quite often that the browser controller fails to kill chrome on the package waterfall.

R=eernst@google.com
BUG=

Review URL: https://codereview.chromium.org//1150543004
diff --git a/tools/task_kill.py b/tools/task_kill.py
index 457e45f..a091164 100755
--- a/tools/task_kill.py
+++ b/tools/task_kill.py
@@ -172,7 +172,9 @@
 
 def KillBrowsers():
   status = Kill('firefox')
-  status += Kill('chrome')
+  # We don't give error on killing chrome. It happens quite often that the
+  # browser controller fails in killing chrome, so we silently do it here.
+  Kill('chrome')
   status += Kill('iexplore')
   status += Kill('safari')
   status += Kill('content_shell')