Fix path previously only intended for Windows. (Now runs also if the out directory doesn't exist).

tools/testing/perf_testing/run_perf_tests.py

Index: tools/testing/perf_testing/run_perf_tests.py
===================================================================
--- tools/testing/perf_testing/run_perf_tests.py	(revision 5994)
+++ tools/testing/perf_testing/run_perf_tests.py	(working copy)
@@ -14,6 +14,7 @@
 from os.path import dirname, abspath
 import platform
 import shutil
+import stat
 import subprocess
 import time
 import traceback
@@ -103,8 +104,9 @@
   # error to be thrown when we use shutil.rmtree. This helper function changes
   # the permissions so we can still delete the directory.
   def on_rm_error(func, path, exc_info):
-    os.chmod(path, stat.S_IWRITE)
-    os.unlink(path)
+    if os.path.exists(path):
+      os.chmod(path, stat.S_IWRITE)
+      os.unlink(path)
   # TODO(efortuna): building the sdk locally is a band-aid until all build
   # platform SDKs are hosted in Google storage. Pull from https://sandbox.
   # google.com/storage/?arg=dart-dump-render-tree#dart-dump-render-tree%2Fsdk