[Android] Fall back to active_devices if the known device list file is empty.

build/android/pylib/perf/setup.py

Index: build/android/pylib/perf/setup.py
diff --git a/build/android/pylib/perf/setup.py b/build/android/pylib/perf/setup.py
index 31db14fa031e0cccfe8b573bb6bc0b56f9dbaaeb..6abefef1f475f45673eea1e560dc4a62a7cff895 100644
--- a/build/android/pylib/perf/setup.py
+++ b/build/android/pylib/perf/setup.py
@@ -28,6 +28,10 @@ def _GetAllDevices(active_devices, devices_path):
     if devices_path:
       devices = [device_utils.DeviceUtils(s)
                  for s in device_list.GetPersistentDeviceList(devices_path)]
+      if not devices and active_devices:
+        logging.warning('%s is empty. Falling back to active devices.',
+                        devices_path)
+        devices = active_devices
     else:
       logging.warning('Known devices file path not being passed. For device '
                       'affinity to work properly, it must be passed.')