Correct invalid use of 'is' when '==' should have been used.

R=nsylvain@chromium.org
BUG=


Review URL: https://chromiumcodereview.appspot.com/11046016

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@159909 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/subprocess2.py b/subprocess2.py
index 043194f..364aace 100644
--- a/subprocess2.py
+++ b/subprocess2.py
@@ -295,9 +295,9 @@
       # This thread needs to be optimized for speed.
       while threads:
         item = queue.get()
-        if item[0] is 'stdout':
+        if item[0] == 'stdout':
           self.stdout_cb(item[1])
-        elif item[0] is 'stderr':
+        elif item[0] == 'stderr':
           self.stderr_cb(item[1])
         else:
           # A thread terminated.