[Telemetry] Add temporary logging to desktop_browser_finder

We've seen scenarios where DesktopBrowserFinder unit tests fail because
sys.platform reports a wrong platform, presumably due to being stubbed out too
aggressively by system_stub. This CL adds some temporary logging to make sure
that the reason browsers sometimes can't be found on the Catapult CQ is due to
this stubbing problem.

BUG=catapult:#3074
NOTRY=true

Review-Url: https://codereview.chromium.org/2567783002

telemetry/telemetry/internal/backends/chrome/desktop_browser_finder.py

@@ -123,6 +123,8 @@ def FindAllBrowserTypes(_):
 
 def FindAllAvailableBrowsers(finder_options, device):
   """Finds all the desktop browsers available on this machine."""
+  logging.warning('sys.platform reports %s to DesktopBrowserFinder',
+                  sys.platform)
   if not isinstance(device, desktop_device.DesktopDevice):
     return []