Bug 1335127 - Fall back to stub IO counters when psutil fails; r=gps

The fallback at [1], when psutil.disk_io_counters() fails, returns a
simple list but callers to get_disk_io_counters() expect an object with
specific properties. Returning a stub object from PsutilStub fixes that.

[1] https://dxr.mozilla.org/mozilla-central/rev/1d025ac534a6333a8170a59a95a8a3673d4028ee/testing/mozbase/mozsystemmonitor/mozsystemmonitor/resourcemonitor.py#67

testing/mozbase/mozsystemmonitor/mozsystemmonitor/resourcemonitor.py

@@ -64,7 +64,7 @@ def get_disk_io_counters():
     try:
         io_counters = psutil.disk_io_counters()
     except RuntimeError:
-        io_counters = []
+        io_counters = PsutilStub().disk_io_counters()
 
     return io_counters